code: public MessageEventDefinitionBuilder messageEventDefinition(String id) {
      MessageEventDefinition messageEventDefinition = createEmptyMessageEventDefinition();
      if (id != null) {
        messageEventDefinition.setId(id);
      }
  
      element.getEventDefinitions().add(messageEventDefinition);
      return new MessageEventDefinitionBuilder(modelInstance, messageEventDefinition);
    }
docs: Creates an empty message event definition with the given id
  and returns a builder for the message event definition.
  
  @param id the id of the message event definition
  @return the message event definition builder object