It shouldn't happen that a view is placed above the shells
it has spawned on sfx2 dispatcher's shell stack 'cause
during dispose, such view gets deleted before the shells
that still refer to it.

Alas it does happen with Wollmux (which makes couple of
Writer UNO API calls before SwView is activated) so guard
against that

Change-Id: Ic7f21992c5e55a48c63dffc733c8f5b0a59a9126
Reviewed-on: https://gerrit.libreoffice.org/50256Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>