[BUGFIX] Fix broken disable overlay in FormEngine

Resolves: #75397
Releases: master, 7.6
Change-Id: I7cb4f04e38e3d9f755aaf92012eb56d71a1ad6f7
Reviewed-on: https://review.typo3.org/48110


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>