instead of returning a mutable ref, which just reads weird

Change-Id: Ia2e2a7222f6ccd73e1d7f69dd7c2523f3a05ae24
Reviewed-on: https://gerrit.libreoffice.org/61309
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>