I cannot give a clear path to reproduce this, since I reproduced this only twice locally, but raw way to achieve this:
1. Open and build a CMake project using Qt different from the system one
2. Close QtC and remove project .user file
3. Run QtC with the -tcs flag (ensure only system Qt is in PATH)
4. Open the project and import its configuration that points to a Qt from installer(or any different, but not the system one)
5. Add one kit that uses system Qt and one kit that uses Qt from installer(or any different, but not the system one)
6. Build the project using both kits one by one
7. Remove the last active kit

The above steps(with slight monkey job by removing and adding kits more than once) lead to the attached stack trace twice in my environment, but cannot say what exactly leads to this.