fdo#86557: svx: fix crash when creating new "XML Form Document"

Strangely it doesn't crash on Linux.

(regression from 713735fc89b9e0222d8b62d051153da6932b3ebd)

Change-Id: Ie29c795f5edec7d8fdf949027359c0c509edb15c
(cherry picked from commit 8505c7970ac05a3fda13789f1b2c50d6d979294e)