[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: 6.2

Change-Id: Iadeeecc3aef92b8bcc4f7fe4e22882d924f10a50
Reviewed-on: https://review.typo3.org/30590
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring