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java.lang.IllegalArgumentException: The observer is null.	at android.database.Observable.unregisterObserver(Observable.java:64)	at android.widget.BaseAdapter.unregisterDataSetObserver(BaseAdapter.java:42)	at android.widget.AbsListView.onDetachedFromWindow(AbsListView.java:2608)	at android.view.View.dispatchDetachedFromWindow(View.java:11972)	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2532)	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2530)	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2530)	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2530)	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2530)	at android.view.ViewGroup.removeAllViewsInLayout(ViewGroup.java:3784)	at android.view.ViewGroup.removeAllViews(ViewGroup.java:3730)