code: def set_message_ref(self, value):
          
          if value is None:
              self.__message_ref = value
          if not isinstance(value, str):
              raise TypeError("MessageRef must be set to a String")
          else:
              self.__message_ref = value
docs: Setter for 'message_ref' field.
          :param value - a new value of 'message_ref' field. Must be either None (message_ref is optional according to
          BPMN 2.0 XML Schema) or String.