Crash on activity finish

[rubenhorn]

After adding a BlurredView, my App crashes when I go to another screen.
Note that I don't call blurView.blurredView(View) yet.

The layout:

<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="false">
        <!-- my layout -->      
    <com.ms_square.etsyblur.BlurringView
        android:id="@+id/blurView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>
</androidx.coordinatorlayout.widget.CoordinatorLayout>

The fragment:

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    // ...
    blurView.blurConfig(myBlurConfig)
}

The stack trace:

2019-12-16 16:59:39.216 21890-21890/? E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.myapp, PID: 21890
    java.lang.RuntimeException: Unable to destroy activity {com.myapp.BlurActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.ViewTreeObserver android.view.View.getViewTreeObserver()' on a null object reference
        at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:4707)
        at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:4730)
        at android.app.servertransaction.DestroyActivityItem.execute(DestroyActivityItem.java:39)
        at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:145)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1947)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7037)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.ViewTreeObserver android.view.View.getViewTreeObserver()' on a null object reference
        at com.ms_square.etsyblur.BlurringView.onDetachedFromWindow(BlurringView.java:105)
        at android.view.View.dispatchDetachedFromWindow(View.java:20067)
        at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:3987)
        at android.view.ViewGroup.removeViewInternal(ViewGroup.java:5588)
        at android.view.ViewGroup.removeViewInternal(ViewGroup.java:5559)
        at android.view.ViewGroup.removeView(ViewGroup.java:5490)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:973)
        at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1303)
        at androidx.fragment.app.FragmentManagerImpl.dispatchStateChange(FragmentManagerImpl.java:2659)
        at androidx.fragment.app.FragmentManagerImpl.dispatchDestroyView(FragmentManagerImpl.java:2638)
        at androidx.fragment.app.Fragment.performDestroyView(Fragment.java:2805)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:954)
        at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1303)
        at androidx.fragment.app.FragmentManagerImpl.dispatchStateChange(FragmentManagerImpl.java:2659)
        at androidx.fragment.app.FragmentManagerImpl.dispatchDestroy(FragmentManagerImpl.java:2644)
        at androidx.fragment.app.FragmentController.dispatchDestroy(FragmentController.java:329)
        at androidx.fragment.app.FragmentActivity.onDestroy(FragmentActivity.java:366)
        at androidx.appcompat.app.AppCompatActivity.onDestroy(AppCompatActivity.java:210)
        at android.app.Activity.performDestroy(Activity.java:7681)
        at android.app.Instrumentation.callActivityOnDestroy(Instrumentation.java:1306)
        at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:4692)

The source:

EtsyBlur/lib/src/main/java/com/ms_square/etsyblur/BlurringView.java

Line 105 in b51c9e8

if (this.blurredView.getViewTreeObserver().isAlive()) {