Broken in 0.8, nullable instance produces missing declaration bug
Original was filed by @webbnh at https://github.com/kaitai-io/kaitai_struct/issues/1207:

> I'm working on a reproducer; meanwhile, see the lobby chat from [here](https://gitter.im/kaitai_struct/Lobby?at=5a80faf118f388e6268f8c59) to [here](https://gitter.im/kaitai_struct/Lobby?at=5a81946cf283b8e5464f7b80).