function addItem($topItem = self::BUTTONS_TOP)
    {
        $item = new Button();
        $item->setSubStyle(Bgs1::BgEmpty);
        if ($topItem == self::BUTTONS_TOP) {
            $this->items[] = $item;
        } else {
            $this->bottomItems[] = $item;
        }
        $this->lastItem = $item;
    }