code: def get_menu_item(self):
          
          if self.modeladmin_instances:
              submenu = SubMenu(self.get_submenu_items())
              return GroupMenuItem(self, self.get_menu_order(), submenu)
docs: Utilised by Wagtail's 'register_menu_item' hook to create a menu
          for this group with a SubMenu linking to listing pages for any
          associated ModelAdmin instances