Navigating to Another Page From TabBarView's child throws an error

I have a TabBar to switch between login and register, both of them are Stateful widgets. When the Navigator is commented, there is no error. If Navigator is not commented the error mentioned below is thrown.

Providing relevant code below -

Flexible(
  child: TabBarView(
    children: [
      Login(), // A Stateful Widget,
      Register(),
    ],
  ),
),

Padding(
  padding: const EdgeInsets.only(top: 20.0),
  child: MaterialButton(
    color: Theme.of(context).accentColor,
    textColor: Colors.white,
    onPressed: loginPressed(context),
    child: Padding(
      padding: const EdgeInsets.all(15.0),
      child: Text('Login'),
    ),
  ),
),

function loginPressed() -

loginPressed(BuildContext context) {
  //Will Be conditions here right now redirection for sake of testing
  Navigator.pushReplacementNamed(context, 'home');
}

Error -

I/flutter ( 4421): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter ( 4421): The following assertion was thrown building Login(dirty, dependencies: [_InheritedTheme,
I/flutter ( 4421): _LocalizationsScope-[GlobalKey#5872b]], state: _LoginState#6fe41):
I/flutter ( 4421): setState() or markNeedsBuild() called during build.
I/flutter ( 4421): This Overlay widget cannot be marked as needing to build because the framework is already in the
I/flutter ( 4421): process of building widgets.  A widget can be marked as needing to be built during the build phase
I/flutter ( 4421): only if one of its ancestors is currently building. This exception is allowed because the framework
I/flutter ( 4421): builds parent widgets before children, which means a dirty descendant will always be built.
I/flutter ( 4421): Otherwise, the framework might not visit this widget during this build phase.
I/flutter ( 4421): The widget on which setState() or markNeedsBuild() was called was:
I/flutter ( 4421):   Overlay-[LabeledGlobalKey<OverlayState>#e1e0c]
I/flutter ( 4421): The widget which was currently being built when the offending call was made was:
I/flutter ( 4421):   Login
I/flutter ( 4421): 
I/flutter ( 4421): The relevant error-causing widget was:
I/flutter ( 4421):   Login file:///D:/flutter_projects/project/lib/screens/login_reg.dart:50:23
I/flutter ( 4421): 
I/flutter ( 4421): When the exception was thrown, this was the stack:
I/flutter ( 4421): #0      Element.markNeedsBuild.<anonymous closure> (package:flutter/src/widgets/framework.dart:3896:11)
I/flutter ( 4421): #1      Element.markNeedsBuild (package:flutter/src/widgets/framework.dart:3911:6)
I/flutter ( 4421): #2      State.setState (package:flutter/src/widgets/framework.dart:1168:14)
I/flutter ( 4421): #3      OverlayState.insertAll (package:flutter/src/widgets/overlay.dart:344:5)
I/flutter ( 4421): #4      OverlayRoute.install (package:flutter/src/widgets/routes.dart:44:24)
I/flutter ( 4421): #5      TransitionRoute.install (package:flutter/src/widgets/routes.dart:181:11)
I/flutter ( 4421): #6      ModalRoute.install (package:flutter/src/widgets/routes.dart:959:11)
I/flutter ( 4421): #7      NavigatorState.pushReplacement (package:flutter/src/widgets/navigator.dart:1883:14)
I/flutter ( 4421): #8      NavigatorState.pushReplacementNamed (package:flutter/src/widgets/navigator.dart:1710:12)
I/flutter ( 4421): #9      Navigator.pushReplacementNamed (package:flutter/src/widgets/navigator.dart:972:34)
I/flutter ( 4421): #10     _LoginState.loginPressed (package:project/screens/login.dart:72:15)
I/flutter ( 4421): #11     _LoginState.build (package:project/screens/login.dart:57:30)
I/flutter ( 4421): #12     StatefulElement.build (package:flutter/src/widgets/framework.dart:4334:27)
I/flutter ( 4421): #13     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4223:15)
I/flutter ( 4421): #14     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
I/flutter ( 4421): #15     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
I/flutter ( 4421): #16     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4381:11)
I/flutter ( 4421): #17     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
I/flutter ( 4421): #18     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
I/flutter ( 4421): #19     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
I/flutter ( 4421): #20     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
I/flutter ( 4421): #21     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
I/flutter ( 4421): #22     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
I/flutter ( 4421): #23     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
I/flutter ( 4421): #24     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
I/flutter ( 4421): #25     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
I/flutter ( 4421): #26     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5445:14)
I/flutter ( 4421): #27     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
I/flutter ( 4421): #28     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
I/flutter ( 4421): #29     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5445:14)
I/flutter ( 4421): #30     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
I/flutter ( 4421): #31     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
I/flutter ( 4421): #32     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
I/flutter ( 4421): #33     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
I/flutter ( 4421): #34     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
I/flutter ( 4421): #35     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
I/flutter ( 4421): #36     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
I/flutter ( 4421): #37     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
I/flutter ( 4421): #38     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
I/flutter ( 4421): #39     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
I/flutter ( 4421): #40     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
I/flutter ( 4421): #41     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
I/flutter ( 4421): #42     ParentDataElement.mount (package:flutter/src/widgets/framework.dart:4617:11)
I/flutter ( 4421): #43     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
I/flutter ( 4421): #44     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
I/flutter ( 4421): #45     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
I/flutter ( 4421): #46     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
I/flutter ( 4421): #47     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
I/flutter ( 4421): #48     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4381:11)
I/flutter ( 4421): #49     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
I/flutter ( 4421): #50     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
I/flutter ( 4421): #51     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
I/flutter ( 4421): #52     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
I/flutter ( 4421): #53     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
I/flutter ( 4421): #54     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
I/flutter ( 4421): #55     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
I/flutter ( 4421): #56     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
I/flutter ( 4421): #57     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
I/flutter ( 4421): #58     SliverMultiBoxAdaptorElement.updateChild (package:flutter/src/widgets/sliver.dart:1288:36)
I/flutter ( 4421): #59     SliverMultiBoxAdaptorElement.createChild.<anonymous closure> (package:flutter/src/widgets/sliver.dart:1273:20)
I/flutter ( 4421): #60     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2412:19)
I/flutter ( 4421): #61     SliverMultiBoxAdaptorElement.createChild (package:flutter/src/widgets/sliver.dart:1266:11)
I/flutter ( 4421): #62     RenderSliverMultiBoxAdaptor._createOrObtainChild.<anonymous closure> (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:354:23)
I/flutter ( 4421): #63     RenderObject.invokeLayoutCallback.<anonymous closure> (package:flutter/src/rendering/object.dart:1823:58)
I/flutter ( 4421): #64     PipelineOwner._enableMutationsToDirtySubtrees (package:flutter/src/rendering/object.dart:875:15)
I/flutter ( 4421): #65     RenderObject.invokeLayoutCallback (package:flutter/src/rendering/object.dart:1823:13)
I/flutter ( 4421): #66     RenderSliverMultiBoxAdaptor._createOrObtainChild (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:343:5)
I/flutter ( 4421): #67     RenderSliverMultiBoxAdaptor.addInitialChild (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:427:5)
I/flutter ( 4421): #68     RenderSliverFixedExtentBoxAdaptor.performLayout (package:flutter/src/rendering/sliver_fixed_extent_list.dart:196:12)
I/flutter ( 4421): #69     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #70     RenderSliverEdgeInsetsPadding.performLayout (package:flutter/src/rendering/sliver_padding.dart:134:11)
I/flutter ( 4421): #71     _RenderSliverFractionalPadding.performLayout (package:flutter/src/widgets/sliver.dart:1165:11)
I/flutter ( 4421): #72     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #73     RenderViewportBase.layoutChildSequence (package:flutter/src/rendering/viewport.dart:410:13)
I/flutter ( 4421): #74     RenderViewport._attemptLayout (package:flutter/src/rendering/viewport.dart:1367:12)
I/flutter ( 4421): #75     RenderViewport.performLayout (package:flutter/src/rendering/viewport.dart:1285:20)
I/flutter ( 4421): #76     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #77     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #78     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #79     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #80     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #81     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #82     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #83     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #84     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #85     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #86     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #87     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #88     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #89     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #90     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #91     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #92     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #93     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #94     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #95     RenderFlex.performLayout (package:flutter/src/rendering/flex.dart:806:17)
I/flutter ( 4421): #96     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #97     RenderPositionedBox.performLayout (package:flutter/src/rendering/shifted_box.dart:392:13)
I/flutter ( 4421): #98     RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #99     MultiChildLayoutDelegate.layoutChild (package:flutter/src/rendering/custom_layout.dart:163:11)
I/flutter ( 4421): #100    _ScaffoldLayout.performLayout (package:flutter/src/material/scaffold.dart:477:7)
I/flutter ( 4421): #101    MultiChildLayoutDelegate._callPerformLayout (package:flutter/src/rendering/custom_layout.dart:232:7)
I/flutter ( 4421): #102    RenderCustomMultiChildLayoutBox.performLayout (package:flutter/src/rendering/custom_layout.dart:391:14)
I/flutter ( 4421): #103    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #104    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #105    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #106    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #107    _RenderCustomClip.performLayout (package:flutter/src/rendering/proxy_box.dart:1232:11)
I/flutter ( 4421): #108    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #109    RenderPadding.performLayout (package:flutter/src/rendering/shifted_box.dart:206:11)
I/flutter ( 4421): #110    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #111    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #112    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #113    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #114    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #115    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #116    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #117    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #118    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #119    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #120    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #121    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #122    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #123    RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter ( 4421): #124    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
I/flutter ( 4421): #125    RenderOffstage.performLayout (package:flutter/src/rendering/proxy_box.dart:3166:14)
I/flutter ( 4421): #126    RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building _OverlayEntry-[LabeledGlobalKey<_OverlayEntryState>#09487](dirty, state: _OverlayEntryState#46afb):
The getter 'status' was called on null.
Receiver: null
Tried calling: status

The relevant error-causing widget was: 
  MaterialApp file:///D:/flutter_projects/project/lib/main.dart:36:12
When the exception was thrown, this was the stack: 
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1      ModalRoute._buildModalBarrier (package:flutter/src/widgets/routes.dart:1305:27)
#2      _OverlayEntryState.build (package:flutter/src/widgets/overlay.dart:169:25)
#3      StatefulElement.build (package:flutter/src/widgets/framework.dart:4334:27)
#4      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4223:15)
...
════════════════════════════════════════════════════════════════════════════════════════════════════

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The method 'drive' was called on null.
Receiver: null
Tried calling: drive<Offset>(Instance of '_ChainedEvaluation<Offset>')
The relevant error-causing widget was: 
  MaterialApp file:///D:/flutter_projects/project/lib/main.dart:36:12
════════════════════════════════════════════════════════════════════════════════════════════════════
I/flutter ( 4421): #127    RenderStack.performLayout (package:flutter/src/rendering/stack.dart:505:15)
I/flutter ( 4421): #128    RenderObject._layoutWithoutResize (package:flutter/src/rendering/object.dart:1584:7)
I/flutter ( 4421): #129    PipelineOwner.flushLayout (package:flutter/src/rendering/object.dart:844:18)
I/flutter ( 4421): #130    RendererBinding.drawFrame (package:flutter/src/rendering/binding.dart:344:19)
I/flutter ( 4421): #131    WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:774:13)
I/flutter ( 4421): #132    RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:283:5)
I/flutter ( 4421): #133    SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1102:15)
I/flutter ( 4421): #134    SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1041:9)
I/flutter ( 4421): #135    SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:957:5)
I/flutter ( 4421): #139    _invoke (dart:ui/hooks.dart:259:10)
I/flutter ( 4421): #140    _drawFrame (dart:ui/hooks.dart:217:3)
I/flutter ( 4421): (elided 3 frames from package dart:async)
I/flutter ( 4421): 
I/flutter ( 4421): ════════════════════════════════════════════════════════════════════════════════════════════════════

Have no idea what has gone wrong.

Solution

In your widget building code, you are calling that loginPressed method. Remember, when you write the following code:

loginPressed(context)

it's actually calling/executing that function. So, you are actually trying to navigate to another page while the current widget is still building. That's the reason why you are getting the exception.

You can fix it in 2 ways:

Method 1:

Change your onPressed code like:

onPressed: () {
   loginPressed(context);
}

Method 2:

Pass the function reference (You don't need to pass the context, the function will already have access to that):

onPressed:loginPressed,

And change the function like:

 loginPressed() {
    Navigator.pushReplacementNamed(context, 'home');
 }