创建了一个工作正常的父管理菜单项后，我无法添加子菜单页。尝试var\\u dump add\\u submenu\\u page（）的返回值时，我得到“null”。

我的职能是：

    /**
     * Register the admin menu page
     * 
     * @since   1.0.0
     */
    public function add_admin_menu() {
        add_menu_page( \'WPQuotes\', \'WPQuotes\', \'manage_options\', \'wp-quotes-plugin\', \'wpq_admin_settings_page_main\', \'dashicons-networking\' );
        add_submenu_page( \'wp-quotes-plugin\', \'New WPQ Form\', \'New WPQ Form\', \'manage-options\', \'wpq-new-form\', \'wpq_admin_settings_page_new_form\');
    }

    public function run_all(){
        add_action( \'admin_menu\', array($this, \'add_admin_menu\'), 10 );
    }