sculpt_update_object now auto-creates a CD_MDISPS layer if missing. Note that it is possible for the depsgraph to provide a multires ccg context without a CD_MDISPS. This causes a PBVH_GRIDS pbvh to be built instead of falling back to PBVH_FACES, which is why this bug happens (of course falling back to PBVH_FACES would still be a bug).