docs: def clean_up_tabs(self):
          """ Method remove state-tabs for those no state machine exists anymore.
          """
          tabs_to_close = []
          for state_identifier, tab_dict in list(self.tabs.items()):
              if tab_dict['sm_id'] not in self.model.state_machine_manager.state_machines:
                  tabs_to_close.append(state_identifier)
          for state_identifier, tab_dict in list(self.closed_tabs.items()):
              if tab_dict['sm_id'] not in self.model.state_machine_manager.state_machines:
                  tabs_to_close.append(state_identifier)
          for state_identifier in tabs_to_close:
              self.close_page(state_identifier, delete=True)