The Pantheon desktop currently has only hard-coded support for screen locking with light-locker, which in turn only works in sessions started by LightDM, and crashes everywhere else.
Since the primary way to get a Pantheon session on fedora is from fedora Workstation (which uses GDM), this isn't a good default experience.
The possible solutions I see are:
1) using gnome-screensaver when starting the Pantheon session from GDM
2) implementing a new, simple screensaver for Pantheon
Since option 2 is probably out of the scope of things that I / we can accomplish for fedora 30, using gnome-screensaver and patching session components to use standard screensaver / screen locking interfaces (instead of hard-coded light-locker interfaces) sounds much more manageable.