xamarin - Android - NullReferenceException on Scroll/Drag-and-Drop (setDragFocus) -

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i'm working on android app using xamarin.android (c#).

i have developed drag-and-drop autoscroll listview, seen in gist.

it works amazingly well, sometimes, sometimes, crashes , leaves no clue why. exception thrown "in unmanaged code" , useful information debugger shows stacktrace:

--- end of managed java.lang.nullpointerexception stack trace --- java.lang.nullpointerexception: attempt invoke virtual method 'void android.view.viewrootimpl.setdragfocus(android.view.view)' on null object reference     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1454)     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1482)     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1482)     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1482)     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1482)     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1482)     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1482)     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1482)     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1482)     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1482)     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1482)     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1482)     @ android.view.viewgroup.dispatchdragevent(viewgroup.java:1482)     @ android.view.viewrootimpl.handledragevent(viewrootimpl.java:5337)     @ android.view.viewrootimpl.access$600(viewrootimpl.java:108)     @ android.view.viewrootimpl$viewroothandler.handlemessage(viewrootimpl.java:3464)     @ android.os.handler.dispatchmessage(handler.java:102)     @ android.os.looper.loop(looper.java:152)     @ android.app.activitythread.main(activitythread.java:5497)     @ java.lang.reflect.method.invoke(native method)     @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:726)     @ com.android.internal.os.zygoteinit.main(zygoteinit.java:616)

i don't suspect can tell me why happening, maybe, there way @ least debug properly?

nowhere, not in log, fault in application, crashes. boom , gone.

the error have null object reference. check object generate problem , watch correctly instantiated.

example

//this object null  object  mymethod(some)  //this object not null  object = new object()  mymethod(some)

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