fdo#73464 - guard against NULL / unset m_xAccessible.

Change-Id: I3351acf18d334447336ee98761190fdff32d5b85
Reviewed-on: https://gerrit.libreoffice.org/7690
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>