Bug 455302

SUMMARY
Dbus paths must match the "[A-Z][a-z][0-9]_" regexp [1]. OTOH hand it is allowed to create seat names like 'seat-foo' which leads to ugly assertion errors in kde applications when doing DBUS calls that involve such paths. 

kscreenlocker is just one of the components effected, another is plasmashell which kickoff somehow referes to. Please feel free to move this bugreport accordingly.

[1] https://dbus.freedesktop.org/doc/dbus-specification.html, section "Valid Object Paths".

STEPS TO REPRODUCE
1. create a seat that contains e.g. '-' in the name
2. login to this seat
3. just try searching something in kickoff menu, or lock a screen and the component will crash

Alternative is to simply do a dbus call that e.g. kickoff is doing when searching for something in the menu:
```"""
dbus-send --print-reply --dest=org.freedesktop.DisplayManager /org/freedesktop/DisplayManager/Seat-ves org.freedesktop.DBus.Introspectable.Introspect
"""

OBSERVED RESULT

Components crashing

EXPECTED RESULT

Unsupported DBUS paths should be validated and errors should be logged or preferably a warning should be shown to the user stating that the name of the seat is invalid. 

I do not have a context wrt. why assertions in the code are made instead, but IMO handling this error gracefully could help novice users debug this problem in the future.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: see the form data I entered
Qt Version: 5.15.3

System is stock Ubuntu 22.04 LTS.

ADDITIONAL INFORMATION