[BUGFIX] Reset FormViewHelper on execution

This commit ensures that the FormViewHelper is in a clean state and
can render the required hidden fields (again).

Resolves: #77097
Releases: master, 7.6, 6.2
Change-Id: I82f90ee7c0aaf44cd48a9abde6dbb012f536543e
Reviewed-on: https://review.typo3.org/49104


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Markus Sommer <markussom@posteo.de>
Tested-by: Markus Sommer <markussom@posteo.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>