multigrid preconditioner do not work for periodic domain

It seems there is a bug for multigrid preconditioner for periodic domain problem...
For the periodic Poisson problem, the local, bddc preconditioner works find, but multigrid preconditioner produces a bug...

Say, I kind of recall algebraic multigrid from hyper was also supported in NGSOLVE, is there an easy way to use it?

The grid transfer operators do don't work for periodic spaces, but I assume we can get them with moderate effort.
I just added a check to throw an exception instead of the core-dump.

Yes, you can use hypre with the MPI-parallel version

The modernization of the built-in AMG is currently work in progress.

Joachim