[BUGFIX] Nested flexforms no longer throw exception

This patch prevents nested flexforms from throwing
an exception on saving or opening the record.

Resolves: #59368
Releases: master, 6.2
Change-Id: Iadeeecc3aef92b8bcc4f7fe4e22882d924f10a50
Reviewed-on: http://review.typo3.org/32675
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>