Merge pull request #594 from contao-community-alliance/hotfix/fix_php…

…warning_treepicker

Fix phpwarning treepicker

src/Contao/View/Contao2BackendView/TreePicker.php

@@ -449,7 +449,7 @@ public function __get($key)
                 return $this->titleIcon;
 
             case 'mandatory':
-                return $this->arrConfiguration['mandatory'];
+                return $this->arrConfiguration['mandatory'] ?? false;
 
             case 'orderField':
                 return $this->orderField;