add BootstrapPlugin::register() to Dropdown widget

Dropdown.php

@@ -46,7 +46,6 @@ class Dropdown extends Widget
      */
     protected $_containerOptions = [];
 
-
     /**
      * Initializes the widget.
      * If you override this method, make sure you call the parent implementation first.
@@ -64,6 +63,7 @@ class Dropdown extends Widget
     public function run()
     {
         echo $this->renderItems($this->items);
+        BootstrapPluginAsset::register($this->getView());
     }
 
     /**