1. Configure an invalid kit in Creator.
   I used a kit which neither has a Qt version nor a compiler. The Options dialog will display a red exclamation mark for this kit.
2. Open a project which does not have user settings.
   Creator reports that no valid kits were found. Still the kit is listed in the bar on the left.
3. Click on the kit's name in that bar.
   Creator will configure the project to use the invalid kit.

I don't think that it makes sense to offer the use of invalid kits.