TYPO3 Core

I want to get my extconf in a postinstall-hook. In advance i'm unsure if this is a bug or just the wrong strategy to get extconf.

My call in ext_conf_template.txt

type=user[EXT:my_ext/Classes/Install/PostInstallHook.php:&Vendor\MyExt\Install\PostInstallHook->setup];

This works and load the "setup"-method. That lines ...

$configurationUtility = new \TYPO3\CMS\Extensionmanager\Utility\ConfigurationUtility();
$configuration = $configurationUtility->getCurrentConfiguration('my_ext');

in the method make the fatal error ...

Fatal error: Call to a member function createArrayFromConstants() on a non-object in /var/source_libs/typo3_src-6.0.0-dev/typo3/sysext/extensionmanager/Classes/Utility/ConfigurationUtility.php on line 97

Please let me know if you need something more.