Fixing the issue #3292

This change fixes the issue #3292. The details were mentioned in the comments to the issue.

ButtonDropdown.php

@@ -64,7 +64,10 @@ class ButtonDropdown extends Widget
      */
     public function run()
     {
-        echo $this->renderButton() . "\n" . $this->renderDropdown();
+        echo Html::beginTag('div', ['class' => 'btn-group']);
+        echo "\n" . $this->renderButton();
+        echo "\n" . $this->renderDropdown();
+        echo "\n" . Html::endTag('div');
         $this->registerPlugin('button');
     }