- The UI-Kit is added by Application Express when creating any new application. If it were added to the PRPC OOTB application, it would be added to all applications, including ones that weren't designed with the UI-Kit in mind.
- If you put it into the implementation layer, the implemention layer app would differ materially from the framework layer app. In essence, you would need to refactor the UI for each implementation. By adding it to the framework layer, it's automatically inherited into each implementation.
This maybe the reason why you are receiving the error. Could you refactor the UI and check?