Avoid leaking dbus objects

Change-Id: Ifb6a7bb3867bba293fd256c87998528a90053507
Reviewed-on: https://gerrit.libreoffice.org/11678
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>