Use default:selection handler

Closes #2987189

src/Entity/PrivateMessageThread.php

@@ -122,7 +122,7 @@ class PrivateMessageThread extends ContentEntityBase implements PrivateMessageTh
    * {@inheritdoc}
    */
   public function getMessages(bool $includeBlocked = FALSE): array {
-    $messages = $this->get('private_messages')->referencedEntities();
+    $messages = array_values($this->get('private_messages')->referencedEntities());
     if ($includeBlocked) {
       return $messages;
     }