docs: def prepare_destruction(self):
          """Get rid of circular references"""
          self._tool = None
          self._painter = None
          self.relieve_model(self._selection)
          self._selection = None
          # clear observer class attributes, also see ExtendenController.destroy()
          self._Observer__PROP_TO_METHS.clear()
          self._Observer__METH_TO_PROPS.clear()
          self._Observer__PAT_TO_METHS.clear()
          self._Observer__METH_TO_PAT.clear()
          self._Observer__PAT_METH_TO_KWARGS.clear()