repredict_only mode breaks stills_process

[dermen]

reproducer for this:

# run a cctbx_project/simtbx test to create a simulated lysozyme image test_full_100.cbf
libtbx.python simtbx/nanoBragg/tst_nanoBragg_cbf_write.py

# process that image
dials.stills_process test_full_100.cbf  indexing.known_symmetry.space_group=P43212 refinement_protocol.mode="repredict_only"

The error is

Couldn't index test_full_100 Python argument types in
    None.None(Experiment, list)
did not match C++ signature:
    None(dxtbx::model::Experiment {lvalue}, std::shared_ptr<dxtbx::model::CrystalBase>)

The culprit appears to be this line from stills_indexer

Possible fix

nv_out = nv()
try:
    assert len(nv_out)==1
    nv_out = nv_out[0]
except Exception:
    pass
experiments[expt_id].crystal = nv_out

but I dont know enough about nv() to do the implementation