This is the message:

ERROR: ../../../../qbs-1.3/src/lib/corelib/buildgraph/buildgraph.cpp:475 ASSERT: !child->product.isNull()

It is unclear how this can ever happen:
a) All code that creates or loads nodes seems to set the product.
b) The same checks are also done before storing a project, and if a problem is found, nothing is stored.
So while it appears impossible that this can happen, I have seen it several times, with the qbs and the Qt Creator projects. No idea how to trigger it, though.