[BUGFIX] Undefined variable $multiSelectId in FormEngine

There is a undefined variable $multiSelectId in FormEngine. This
is introduced with #46357

Resolves: #56457
Releases: 6.2
Change-Id: I88fd4f9b36e6421b338011a1cc31c22987019dc9
Reviewed-on: https://review.typo3.org/27959
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel