method check for remote videos (#162 An error occurred )

* method check for remote videos

* merged upstream changes

* fixed syntax

src/EventSubscriber/MediaLinkHeaderSubscriber.php

@@ -71,7 +71,8 @@ class MediaLinkHeaderSubscriber extends LinkHeaderSubscriber implements EventSub
     $source_field = $type_configuration['source_field'];
 
     if (empty($source_field) ||
-        !$media->hasField($source_field)
+      !$media->hasField($source_field) ||
+      !method_exists($media->get($source_field), 'referencedEntities')
     ) {
       return $links;
     }