Resolves: fdo#71106 editing dde function crashes

Change-Id: Id2f4bffb7de7201ddeb8f75daaa2723cc42c59b0
(cherry picked from commit 3b27470860cfa377dfc0cb940bba204d827434ad)
Reviewed-on: https://gerrit.libreoffice.org/6520
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>