I am not sure what you are trying to do here. But as far as knowing when the View of a MenuPane s destroyed is simple.
On AttachEvent of a MenuPane you can add code to add a DestroyNotify event on to the View of the MenuPane. Now when this view is destroyed the event handler will be called.
Another way, that may work (I have not tested) is to add a DetachEvent handler to the MenuPane and use it for your purpose.