code: public void addSubMenuItem(CmsContextMenuItemWidget contextMenuItem) {
  
          if (!hasSubMenu()) {
              m_subMenu = new CmsContextMenuOverlay();
              m_subMenu.setOwner(m_rootComponent.getExtensionTarget());
              setStylePrimaryName("v-context-menu-item-basic-submenu");
          }
  
          contextMenuItem.setParentItem(this);
          m_subMenu.addMenuItem(contextMenuItem);
      }
docs: Adds given context menu item into the sub menu of this item.<p>
  
  @param contextMenuItem the menu item