Disposing front stack pages on mobile

[c-ramarin]

When I call Frame.ForwardStack.Clear(), sometimes the Android app crashes with this exception:

Java.Lang.NullPointerException
Message=Attempt to read from field 'int android.view.View.mViewFlags' on a null object reference in method 'void android.view.ViewGroup.dispatchDraw(android.graphics.Canvas)'

0xFFFFFFFFFFFFFFFF in Android.Runtime.RuntimeNativeMethods.monodroid_debugger_unhandled_exception	
0x59 in Android.Runtime.JNIEnv.PropagateUncaughtException at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.Runtime/JNIEnv.cs:125,5

This occurs after unloaded is called on those front stack pages. Anyone know what is going on? Thank you.

[darenm]

@jeromelaban Any insight here?

[jeromelaban]

This is not something we've seen. A repro would help, and if possible a larger stack that you be able to get when running without the debugger.

[c-ramarin]

I got it from Logcat:

FATAL EXCEPTION: main
                                                                                                    Process: kahua.host.mobile.android, PID: 15165
                                                                                                    java.lang.NullPointerException: Attempt to read from field 'int android.view.View.mViewFlags' on a null object reference in method 'void android.view.ViewGroup.dispatchDraw(android.graphics.Canvas)'
                                                                                                    	at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4316)
                                                                                                    	at android.view.View.draw(View.java:23892)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22756)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
                                                                                                    	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
                                                                                                    	at android.view.View.updateDisplayListIfDirty(View.java:22712)
                                                                                                    	at android.view.ThreadedRenderer.updateViewTreeDisplayList(ThreadedRenderer.java:694)
                                                                                                    	at android.view.ThreadedRenderer.updateRootDisplayList(ThreadedRenderer.java:700)
                                                                                                    	at android.view.ThreadedRenderer.draw(ThreadedRenderer.java:798)
                                                                                                    	at android.view.ViewRootImpl.draw(ViewRootImpl.java:4939)
                                                                                                    	at android.view.ViewRootImpl.performDraw(ViewRootImpl.java:4643)
                                                                                                    	at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:3822)
                                                                                                    	at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:2465)
                                                                                                    	at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:9305)
                                                                                                    	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1339)
                                                                                                    	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1348)
                                                                                                    	at android.view.Choreographer.doCallbacks(Choreographer.java:952)
                                                                                                    	at android.view.Choreographer.doFrame(Choreographer.java:882)

[c-ramarin]

Another longer crash log:

[AndroidRuntime] Shutting down VM
[AndroidRuntime] FATAL EXCEPTION: main
[AndroidRuntime] Process: kahua.host.mobile.android, PID: 16197
[AndroidRuntime] java.lang.NullPointerException: Attempt to read from field 'int android.view.View.mViewFlags' on a null object reference in method 'void android.view.ViewGroup.dispatchDraw(android.graphics.Canvas)'
[AndroidRuntime] 	at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4316)
[AndroidRuntime] 	at android.view.View.draw(View.java:23892)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22756)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ViewGroup.recreateChildDisplayList(ViewGroup.java:4540)
[AndroidRuntime] 	at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:4513)
[AndroidRuntime] 	at android.view.View.updateDisplayListIfDirty(View.java:22712)
[AndroidRuntime] 	at android.view.ThreadedRenderer.updateViewTreeDisplayList(ThreadedRenderer.java:694)
[AndroidRuntime] 	at android.view.ThreadedRenderer.updateRootDisplayList(ThreadedRenderer.java:700)
[AndroidRuntime] 	at android.view.ThreadedRenderer.draw(ThreadedRenderer.java:798)
[AndroidRuntime] 	at android.view.ViewRootImpl.draw(ViewRootImpl.java:4939)
[AndroidRuntime] 	at android.view.ViewRootImpl.performDraw(ViewRootImpl.java:4643)
[AndroidRuntime] 	at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:3822)
[AndroidRuntime] 	at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:2465)
[AndroidRuntime] 	at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:9305)
[AndroidRuntime] 	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1339)
[AndroidRuntime] 	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1348)
[AndroidRuntime] 	at android.view.Choreographer.doCallbacks(Choreographer.java:952)
[AndroidRuntime] 	at android.view.Choreographer.doFrame(Choreographer.java:882)
[AndroidRuntime] 	at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1322)
[AndroidRuntime] 	at android.os.Handler.handleCallback(Handler.java:958)
[AndroidRuntime] 	at android.os.Handler.dispatchMessage(Handler.java:99)
[AndroidRuntime] 	at android.os.Looper.loopOnce(Looper.java:205)
[AndroidRuntime] 	at android.os.Looper.loop(Looper.java:294)
[AndroidRuntime] 	at android.app.ActivityThread.main(ActivityThread.java:8177)
[AndroidRuntime] 	at java.lang.reflect.Method.invoke(Native Method)
[AndroidRuntime] 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:552)
[AndroidRuntime] 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:971)
**Java.Lang.NullPointerException:** 'Attempt to read from field 'int android.view.View.mViewFlags' on a null object reference in method 'void android.view.ViewGroup.dispatchDraw(android.graphics.Canvas)''

[jeromelaban]

This looks like everything is in the native stack. If will be important to have a repro so we can take a look.