check if variable exists before accessing it, fixes #4480

Closes #4480

Merge request studip/studip!3265

lib/classes/AutoInsert.php

@@ -111,7 +111,7 @@ class AutoInsert
         foreach ($domains as $domain) {
 
             $key = $domain . '.' . $status;
-            if (is_array($this->settings[$key])) {
+            if (isset($this->settings[$key]) && is_array($this->settings[$key])) {
                 foreach ($this->settings[$key] as $id => $value) {
                     $settings[$id] = $value;
                 }