When you open the screen locker settings in systemsettings5, there's no possibility to change the theme. The corresponding area is just blank.

Reproducible: Always

Steps to Reproduce:
1. Open the screen locker module in systemsettings5 or run "kcmshell5 screenlocker"


Actual Results:  
No possibility to change theme.

Expected Results:  
A list of available themes should be shown at the bottom.

If you enter the "Workspace Theme"->"Look And Feel" module (which has a similar theme selection widget), it starts to work in the "Screen Locker" module too.