the dialog hierarchy changed iwith the ui format, fdo#81867

We need to be explicit now and can no longer rely on GetParent returning
the right object. Seems like sometimes you get punished for an ugly hack
a bit later than expected.

Change-Id: I145e97ae0ceee741c4128c29a6ca26fad2178cb5
(cherry picked from commit fe5463fb7bc740bfec5d2eb7408b8ff3624d8c31)
Reviewed-on: https://gerrit.libreoffice.org/10653
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>