VirtualFolderType::__get: check for existance before returning the value of...

VirtualFolderType::__get: check for existance before returning the value of the requested array index, fixes #2974

lib/filesystem/VirtualFolderType.php

@@ -87,7 +87,7 @@ class VirtualFolderType implements FolderType
      */
     public function __get($attribute)
     {
-        return $this->folderdata[$attribute];
+        return $this->folderdata[$attribute] ?? null;
     }
 
     /**