public function renderWidget()
    {
        // BEGIN:nestable-box
        echo Html::beginTag('div', ['class' => 'nestable-box']);
        foreach ($this->items as $item) {
            $this->renderGroup($item);
        }
        // END:nestable-box
        echo Html::endTag('div');
        if ($this->hasModel()) {
            echo Html::activeHiddenInput($this->model, $this->attribute);
        } else {
            echo Html::hiddenInput($this->name);
        }
    }