[TYPO3-mvc] addUsergroup in extended feUser

[Domi]

Hello

I found now the right error log, it was hidden because of my vhost config:

PHP Fatal error:  Call to a member function attach() on a non-object in 
/data/www/websites/studentsjiwa/typo3/sysext/extbase/Classes/Domain/Model/FrontendUser.php 
on line 215, referer: 
http://localhost/websites/studentsjiwa/index.php?id=1108&XDEBUG_SESSION_START=12921

in my function:
public function addUsergroup(Tx_Extbase_Domain_Model_FrontendUserGroup 
$usergroup) {
		$this->usergroup->attach($usergroup);
	}

Weird is, that in the debug I get $usergroup as an object of 
Tx_Extbase_Domain_Model_FrontendUserGroup with exactly the values 
inside. Very Strange, xDebug gives me then somehow a wrong information. 
Still I don't have a clue how to get the "right" object then.

Greetings,
Dominic