protected function toList(array $items = [], $level = 0)
    {
        $prefix = str_repeat('&nbsp;&nbsp;', $level);

        $pages = [];

        foreach ($items as $item) {
            $pages[$item['id']] = $prefix . '' . $item['title'];

            if ($item['children']) {
                $pages += $this->toList($item['children'], $level + 1);
            }
        }

        return $pages;
    }