> Then we should find what callable is NULL and fix the place where it is called. _PyObject_FastCallDict() should never be called with NULL.

My understanding is that the interpreter is being shutdown and half of the objects are freed. We're still holding a reference to *something* in Python space and try calling it.  The obvious fix for that is simply avoid capturing tracebacks if a Future object is created during finalization of the interpreter (it's pointless to capture it anyways at that point).