[BUGFIX] Add missing selector to method adding allowed element support

Change-Id: Ia8784c2ba51232d9e06abb6a36f110b8c4ab2363
Fixes: #52078
Reviewed-on: https://review.typo3.org/24153
Reviewed-by: Jo Hasenau
Tested-by: Jo Hasenau
Reviewed-on: https://review.typo3.org/25130