Not able to include a widget in the scaffold due to missing preferred size constraint?

Question

Getting the following error when implementing my appbar with Flutter. I can include it elsewhere in the children array of the body Stack, just not with the appBar: under Scaffold.

The argument type 'Widget' can't be assigned to the parameter type 'PreferredSizeWidget?'.

I can add the Widget like this and it works, but not like this and I'm trying to figure out why I can't include my appbar() widget in this manner.

This works ok

class _MainPageState extends State<MainPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text("Welcome"),
        centerTitle: true,
        backgroundColor: Colors.transparent,
        elevation: 0.0,
      ),
      body: Stack(
        children: <Widget>[

This does not, though by rights it should as it is merely returning the AppBar when called. Wrapping it in a PreferredSize() like so also does not work.

This doesn't work

Widget appBar() {
  return PreferredSize(
        preferredSize: const Size.fromHeight(100),
        child: Container(color: Colors.transparent, child: AppBar(
    title: const Text("Welcome"),
    centerTitle: true,
    backgroundColor: Colors.transparent,
    elevation: 0.0,
  )));
}

This doesn't work

Widget appBar() {
  return AppBar(
    title: const Text("Welcome"),
    centerTitle: true,
    backgroundColor: Colors.transparent,
    elevation: 0.0,
  );
}

Including appBar() below in the Scaffold

class _MainPageState extends State<MainPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: appBar(),
      body: Stack(
        children: <Widget>[

Answer 1

Replace the return type widget to PreferredSize.

  PreferredSize appBar() {
    return PreferredSize(
        preferredSize: const Size.fromHeight(100),
        child: Container(
            color: Colors.transparent,
            child: AppBar(
              title: const Text("Welcome"),
              centerTitle: true,
              backgroundColor: Colors.transparent,
              elevation: 0.0,
            )));
  }

Also, can be done like appBar: appBar() as PreferredSize but better to do change return type,