public function maybe_add_submenu() {

    // Maybe add submenu.
    if ( isset( $this->menu_ops['submenu'] ) && is_array( $this->menu_ops['submenu'] ) ) {
      $menu = wp_parse_args(
        $this->menu_ops['submenu'],
        [
          'parent_slug' => '',
          'page_title'  => '',
          'menu_title'  => '',
          'capability'  => 'edit_theme_options',
        ]
      );

      $this->pagehook = add_submenu_page( $menu['parent_slug'], $menu['page_title'], $menu['menu_title'], $menu['capability'], $this->page_id, [ $this, 'admin' ] );
    }

  }