AndroidStudio - CompileError: (App keeps stopping) -> RuntimeException: Unable to instantiate activity

Question

I suddenly have the problem that Android Studio doesn't want to compile my projects anymore. The VD is started and there are no other syntax errors specified, but the app closes continuously. Even if I create a new project and change a little thing there, nothing works anymore.

I already searched for the error, but didn't understand exactly which settings were meant, because nobody specified anything. I am relatively new, so I would be very happy if someone could explain this to me in a comprehensible and detailed way - thank you!

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: kuehl.leon.tictactoe, PID: 2719
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{kuehl.leon.tictactoe/kuehl.leon.tictactoe.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2718)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6541)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
        at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:117)
        at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:149)
        at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:29)
        at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:54)
        at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:31)
        at android.support.v7.app.AppCompatDelegateImplN.<init>(AppCompatDelegateImplN.java:31)
        at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:198)
        at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:183)
        at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:519)
        at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190)
        at kuehl.leon.tictactoe.MainActivity.<init>(MainActivity.java:13)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1173)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2708)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) 
        at android.os.Handler.dispatchMessage(Handler.java:105) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6541) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) 

I just want my stuff to get compiled right again!

Answer 1

This error indicates that you are calling findViewById() before setContentView() is called.

Is it possible that you are doing something like that?

public class MainActivity extends AppCompatActivity {

    TextView label = (TextView) findViewById(R.id.label);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

It would help to identify the problem if you could share your activity's code.