Resolve #5100 "Nach Auswahl des Sichtbarkeitsstatus wird nur eine leere Seite angezeigt"

Closes #5100

Merge request studip/studip!3818

lib/middleware/SeminarOpenMiddleware.php

@@ -290,10 +290,13 @@ final class SeminarOpenMiddleware implements MiddlewareInterface
             && $GLOBALS['user']->visible === 'unknown'
         ) {
             require_once('lib/user_visible.inc.php');
+            $html = first_decision($GLOBALS['user']->id);
+            if ($html) {
                 $response = $this->response_factory->createResponse(200);
-            $response->getBody()->write((string)first_decision($GLOBALS['user']->id));
+                $response->getBody()->write($html);
                 return $response;
             }
+        }
 
         return $handler->handle($request);
     }