code: def get_rich_menu_id_of_user(self, user_id, timeout=None):
          
          response = self._get(
              '/v2/bot/user/{user_id}/richmenu'.format(user_id=user_id),
              timeout=timeout
          )
  
          return response.json.get('richMenuId')
docs: Call get rich menu ID of user API.
  
          https://developers.line.me/en/docs/messaging-api/reference/#get-rich-menu-id-of-user
  
          :param str user_id: IDs of the user
          :param timeout: (optional) How long to wait for the server
              to send data before giving up, as a float,
              or a (connect timeout, read timeout) float tuple.
              Default is self.http_client.timeout
          :type timeout: float | tuple(float, float)
          :rtype: str
          :return: rich menu id