Neutron beam setups crash
In setups with neutron beams the neutron is initialised in the beam handler, not in Ahadic. This initialisation is using a wrong spin (0). Instead, it should be the same as in Ahadic. When encountering decays that involve neutrons and have matrix elements implemented, this will lead to crashes because of a mismatch of the expected number of helicity combinations in the ME.
Fun fact: this bug has been around for 12 years (e65e898c), but was apparently only recently exposed when adding matrix elements for some baryonic heavy-flavour decays (42a9ca83 I think).