fix missing code for infinite dropdown

extensions/bootstrap/Dropdown.php

@@ -95,7 +95,7 @@ class Dropdown extends Widget
             $content = Html::a($label, ArrayHelper::getValue($item, 'url', '#'), $linkOptions);
             if (!empty($item['items'])) {
                 unset($this->_containerOptions['id']);
-                $this->renderItems($item['items']);
+                $content .= $this->renderItems($item['items']);
                 Html::addCssClass($options, 'dropdown-submenu');
             }
             $lines[] = Html::tag('li', $content, $options);