fdo#82260 Fix OS X font dropdown crash

Without this, font dropdowns intermittently crash after a window
is moved to another screen

Change-Id: I1af5d8224fd164b73c43a9655fbde364f530c979
Reviewed-on: https://gerrit.libreoffice.org/11105
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>