[BUGFIX] Wrong content element rendering in system extension form

The ability to render custom content elements in the system extension form fails.

Change-Id: Ib2ff3cc391705be458140c9a0dea0e6255948e83
Resolves: #30088
Releases: 4.6
Reviewed-on: http://review.typo3.org/5117
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader