copage: fixed Undefined array key

Author: alex40724

Services/COPage/PC/MediaObject/class.ilPCMediaObjectGUI.php

@@ -1320,7 +1320,7 @@ public function getStyleInput(): ilAdvSelectInputGUI
 
         $chars = $this->getCharacteristics();
         if (is_object($this->content_obj)) {
-            if ($chars[$selected] == "" && ($this->content_obj->getClass() != "")) {
+            if (($chars[$selected] ?? "") == "" && ($this->content_obj->getClass() != "")) {
                 $chars = array_merge(
                     array($this->content_obj->getClass() => $this->content_obj->getClass()),
                     $chars