TYPO3 Core

Hi all,

I have the following element in a flexform:

<type>group</type>
<internal_type>folder</internal_type>
<size>1</size>
<minitems>0</minitems>
<maxitems>1</maxitems>

If I click on the folder-icon in the backend, I get the following message:

Uncaught TYPO3 Exception
#1: PHP Catchable Fatal Error: Argument 1 passed to TYPO3\CMS\Recordlist\Browser\ElementBrowser::TBE_expandSubFolders() must be an instance of TYPO3\CMS\Core\Resource\Folder, null given, called in [My-Document-Root]\typo3\sysext\recordlist\Classes\Browser\ElementBrowser.php on line 1278 and defined in [My-Document-Root]\typo3\sysext\recordlist\Classes\Browser\ElementBrowser.php line 1453 (More information)

TYPO3\CMS\Core\Error\Exception thrown in file
[My-Document-Root]\typo3\sysext\core\Classes\Error\ErrorHandler.php in line 108.

Version: TYPO3 6.1.6

Thank you!
Peter