Resolved: #124231# field variable dialog: check access...

to current field as it could be unavailable caused by its change and a followed
removal from the document by e.g. a conditional hide of a section

(cherry picked from commit 58f0d79a20c3b9683c2d6a4be5dde14fe7688e01)
(cherry picked from commit 619fb83a3bc6d372632d382eaa1eb94fb9a8e8e9)

Conflicts:
	sw/source/ui/fldui/fldvar.cxx

Change-Id: I3f37b97ca8245c1996790622dd25437906b080db