Replacement Kits can interfere with ability to Configure Project

This problem only happened when I had installed Qt 5.15.2 after deleting Qt 5.12.10. Note that I had previously built some projects in 5.12.10 that were still on disk.

The problem I saw is that leftover Replacement Kits from 5.12.10 were somehow blocking QtCreator's ability to successfully Configure Project using the new 5.15.2 Auto-detected Default kit.

In more detail: After the Configure Project step, I could see a problem at the top-left of the Project pane: instead of the usual "purple gear" showing on the .pro, it just showed the name of the .pro file, with a '>' to the left of it. The .pro could not be expanded to show the contents of the project; only the name of the .pro file was shown. The configure had failed, it didn't show any of the contents of the project, and in the lower left pane of Qt Creator, near the build transport controls, it showed as "Unconfigured".

I was able to workaround the problem. I decided to delete all the Replacement Kits, which I did not need anyway. After that time, Configure Project started working again. All of a sudden, QtCreator seemed to be able to able to use the 5.15.2 kit correctly.