nonlinear elasticity tutorial fails at mat.Inverse step (Null communicator)

[davydden]

https://ngsolve.org/docu/latest/whetting_the_appetite/elasticity.html fails at

inv = a.mat.Inverse(fes.FreeDofs() )

with 6.2.2006

$python elasticity.py                 
 Generate Mesh from spline geometry
 Boundary mesh done, np = 68
...
Newton iteration 0
energy =  8.75
Assemble linearization
assemble VOL element 168/168                                 
Fatal error in PMPI_Comm_rank: Invalid communicator, error stack:
PMPI_Comm_rank(111): MPI_Comm_rank(MPI_COMM_NULL, rank=0x236a700) failed
PMPI_Comm_rank(68).: Null communicator
[unset]: write_line error; fd=-1 buf=:cmd=abort exitcode=873025029

this happens for the software stacks built with either openmpi or mpich, thus unlikely to be due to the specific MPI being used.

[davydden]

After some experiments, I noticed that this happens when ngsolve is built with mumps. It could be an issue on mumps side or on ngsolve. I am not too familiar with debugging python wrappers to c++ libs, if you could provide me some guidance I can try to figure out what's going on here...