Adding a Flow to a Mantaflow domain could sometimes cause a crash. This
was because the grids are allocated lazily, and as such, getting them
through `PyObject_GetAttrString` will fail when they are not yet
available. As the resulting Python errors were not cleared, Python could
be left in a bad state, leading to crashes. This is avoided by clearing
these errors before returning from `callPythonFunction` when such an
error is raised.
Ref !141364
94f4df8ec4