code: public static CmsGroupContainerEditor openGroupcontainerEditor(
          CmsGroupContainerElementPanel groupContainer,
          CmsContainerpageController controller,
          CmsContainerpageHandler handler) {
  
          // making sure only a single instance of the group-container editor is open
          if (INSTANCE != null) {
              CmsDebugLog.getInstance().printLine("group-container editor already open");
          } else {
              CmsGroupContainerEditor editor = new CmsGroupContainerEditor(groupContainer, controller, handler);
              RootPanel.get().add(editor);
              editor.openDialog(Messages.get().key(Messages.GUI_GROUPCONTAINER_CAPTION_0));
              editor.getGroupContainerWidget().refreshHighlighting();
              INSTANCE = editor;
          }
          return INSTANCE;
      }
docs: Opens the group container editor.<p>
  
  @param groupContainer the group container
  @param controller the container page controller
  @param handler the container page handler
  
  @return the editor instance