Administration: Fix new item group table array access

[mjansenDatabay]

See: https://mantis.ilias.de/view.php?id=37799

if approved, this has to be picked to release_9 and trunk.

[nicoroeser]

Thank you, @mjansenDatabay!

This patch looks good to me on first sight. @alex40724 is assigned to the Mantis issue report and responsible for that code part, so he’ll probably comment on this, merge, etc.

[alex40724]

Thanks, fixed differently.