code: def exit_code(self) -> Any:
          
          if self._done() or self.is_killed():
              return self._exit_code
          else:
              raise BehaviourNotFinishedException
docs: Returns the exit_code of the behaviour.
          It only works when the behaviour is done or killed,
          otherwise it raises an exception.
  
          Returns:
            object: the exit code of the behaviour