rB67e23b4b2967 revealed the bug. But the bug already existed before,
it just wasn't triggered.
Apparently the problem happens because the python code generated in
`initGuiding()` cannot be executed twice.
The second time the `initGuiding()` code is executed, the local python
variables are removed to make way for the others, but the reference to
one of the grids in a `Solver` object (name='solver_guiding2') is still
being used somewhere. So an error is raised and a crash is forced.
The solution is to prevent the python code in `initGuiding()` from being
executed twice.
When `FLUID_DOMAIN_ACTIVE_GUIDE` is in `fds->active_fields` this
indicates that the pointer in `mPhiGuideIn` has been set and the guiding
is already computed (does not need to be computed again).
Maniphest Tasks: T102257
Differential Revision: https://developer.blender.org/D16416
edc00429e8