With the file, with no root_type Motion; specified

table Motion {
  x: float;
  y: float;
}

calling flatc --lua test.fbs crashes trying to read null.
Exception happens in base.h:ReadScalar(const void *p) where its p is 0. Coming from GetVTable, from name()

I would have expected an error "expected root_type".

However, C++ and Rust successfully generate, so maybe it should be doing whatever it is that they do instead.
Is it supposed to be optional?