docs: def get_state_machine_selection(self):
          """Getter state machine selection
  
          :return: selection object, filtered set of selected states
          :rtype: rafcon.gui.selection.Selection, set
          """
          if self._selected_sm_model:
              return self._selected_sm_model.selection, self._selected_sm_model.selection.states
          else:
              return None, set()