prevent warning, fixes #4778

Closes #4778

Merge request studip/studip!3574

lib/models/BlubberThread.php

@@ -428,11 +428,11 @@ class BlubberThread extends SimpleORMap implements PrivacyObject
             return mb_substr($names, 0, 60);
         }
 
-        if($this['context_type'] === 'course') {
+        if ($this['context_type'] === 'course') {
             if ($this['content']) {
                 return mb_substr((string) Course::find($this['context_id'])->name . ': ' . $this['content'], 0, 50) . ' ...';
             } else {
-                return (string) Course::find($this['context_id'])->name;
+                return (string) (Course::find($this['context_id'])->name ?? _('unbekannt'));
             }
         }