I often get the message when I switch the kit, build configuration within QtCreator, or when changing one of the qbs files.

It is the assert in buildgraph.cpp line 637, I've tracked it to the artifact "Qt.core" for a product, which is disabled because it depends on a product, which again is disabled because it depends on a non existent product.

My solution always was to delete the .bg file and rebuild my project. Now that I have debugged where it comes from I will completely remove the disabled product.