Resolves: fdo#58094 don't crash on attempting mml import

Change-Id: I812f2b3e333ea59e9ebfc75a854faca050af8711
(cherry picked from commit 603e9338568c29c01f812c4fd47b7e8138e015bb)
Reviewed-on: https://gerrit.libreoffice.org/6590
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>