code: @Override
      public void add(Widget widget) {
          if (widget instanceof HasJqmHeader) {
              setHeader((HasJqmHeader) widget);
              return;
          } else if (widget instanceof HasJqmFooter) {
              setFooter((HasJqmFooter) widget);
              return;
          } else if (widget instanceof JQMContent) {
              throw new RuntimeException("Do not add content widgets here, call createContent() instead");
          }
          getPrimaryContent().add(widget);
  
          // if page is already enhanced then we need to enhance the content manually
          // if (enhanced) triggerCreate();
      }
docs: Adds a widget to the primary content container of this page.
  
  @param widget the widget to add to the primary content