prevent php-warnings, closes #3497

Closes #3497

Merge request studip/studip!2391

lib/classes/ActionMenu.php

@@ -285,7 +285,7 @@ class ActionMenu
     protected function getKeyForIndex($index)
     {
         foreach ($this->actions as $key => $value) {
-            if ($value['index'] === $index) {
+            if (!empty($value['index']) && $value['index'] === $index) {
                 return $key;
             }
         }