[BUGFIX] Check if selected page is available before loading BE module

If a formerly selected page (in page tree) is not available anymore
(e.g. deleted), any access to a backend module will now check
this condition and will avoid an error.
An empty module will be displayed in such a case.

Resolves: #66449
Releases: master, 7.6
Change-Id: I4caf6abf715af2009705caf126f2614ae5cde9ed
Reviewed-on: https://review.typo3.org/44923


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>