Called on panel destruction.

def OnDestroy(self, event):
        """Called on panel destruction."""
        # deregister observers
        if hasattr(self, 'cardmonitor'):
            self.cardmonitor.deleteObserver(self.cardtreecardobserver)
        if hasattr(self, 'readermonitor'):
            self.readermonitor.deleteObserver(self.readertreereaderobserver)
            self.cardmonitor.deleteObserver(self.readertreecardobserver)
        event.Skip()