[BUGFIX] EXT:form - load custom form editor templates

Avoid javascript errors if a custom form editor templates is used.

Resolves: #79184
Releases: master
Change-Id: I5a769234dc9088e34b2ab69cf82ac613fcf82210
Reviewed-on: https://review.typo3.org/51163


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>