Unexpected problem when unmounting

I've got this error reported from an app in production and it seems that it tried to remove view more than once. Maybe a simple try/catch on [L138](https://github.com/zserge/anvil/blob/master/anvil/src/main/java/trikita/anvil/Anvil.java#L138) would solve with no side effects.

Fatal Exception: java.lang.IllegalArgumentException
The observer is null.

android.database.Observable.unregisterObserver (Observable.java:64)
android.widget.BaseAdapter.unregisterDataSetObserver (BaseAdapter.java:42)
android.widget.AbsListView.onDetachedFromWindow (AbsListView.java:2668)
android.view.View.dispatchDetachedFromWindow (View.java:12024)
android.view.ViewGroup.dispatchDetachedFromWindow (ViewGroup.java:2612)
android.view.ViewGroup.dispatchDetachedFromWindow (ViewGroup.java:2610)
android.view.ViewGroup.removeViewsInternal (ViewGroup.java:3792)
android.view.ViewGroup.removeViews (ViewGroup.java:3674)
**trikita.anvil.Anvil.unmount (Anvil.java:138)**