Update BeautifyResponseFilter.php

src/EventSubscriber/BeautifyResponseFilter.php

@@ -69,7 +69,7 @@ class BeautifyResponseFilter implements EventSubscriberInterface {
 
     $response = $event->getResponse();
     $content_type = $response->headers->get('Content-Type');
-    if (stripos($content_type, 'text/html') === FALSE) {
+    if (stripos((string) $content_type, 'text/html') === FALSE) {
       return;
     }