protected function renderSides($sides)
    {
        $lines = [];
        $lines[] = Html::beginTag('div', $this->sidesOptions);
        foreach($sides as $side) {
            if(!array_key_exists('content', $side)) {
                throw new InvalidConfigException("The 'content' option is required per sides");
            }
            $options = ArrayHelper::getValue($side, 'options', []);
            Ui::addCssClass($options, 'side');
            $active = ArrayHelper::getValue($side, 'active', false);
            if($active === true) {
                Ui::addCssClass($options, 'active');
            }
            $lines[] = Html::tag('div', $side['content'], $options);

        }
        $lines[] = Html::endTag('div');

        return implode("\n", $lines);
    }