Fixed on master by 7aeced1f, I’m glad I took care of it (it wasn’t crashing yet at that time). It’s an old error, but the update of a lib (GLib ?) must have woken it up
There should be a version 4.18.0 in not too long I think.
so check the git AUR build:
https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=xfce4-screensaver-git
should include this change if you freshly build directly from git