code: public void assertExecuteMethodResponseDefined(ActionResponse response) {
          if (response.isUndefined()) {
              final ExceptionMessageBuilder br = new ExceptionMessageBuilder();
              br.addNotice("Cannot return undefined resopnse from the execute method.");
              br.addItem("Advice");
              br.addElement("Not allowed to return undefined() in execute method.");
              br.addElement("If you want to return response as empty body,");
              br.addElement("use asEmptyBody() like this:");
              br.addElement("  @Execute");
              br.addElement("  public HtmlResponse index() {");
              br.addElement("      return HtmlResponse.asEmptyBody();");
              br.addElement("  }");
              br.addItem("Action Execute");
              br.addElement(execute);
              final String msg = br.buildExceptionMessage();
              throw new ExecuteMethodReturnUndefinedResponseException(msg);
          }
      }
docs: ===============