TYPO3 Core

If you define a select field as renderMode=Tree in a section of a flexform, there's an error :

Invalid argument supplied for foreach() in typo3/sysext/backend/Classes/Form/Element/SelectTreeElement.php line 51

My solution was :
line 51

- $selectedNodes = ($parameterArray['itemFormElValue']);
+ $selectedNodes = (empty($parameterArray['itemFormElValue'])) ? array() : $parameterArray['itemFormElValue'];

But I don't know why this array was empty ...