With several sampler candidates the inspector first drops stages whose
outputs reach only previews. That pass looked each API graph node's
`class_type` up in the preview class set, so a list or object
`class_type` raised TypeError and the inspect route answered 500. Only
a string names a class now; such a node is an ordinary terminal, as the
rest of the resolver already treats it.
Vito Sansevero
564049b373