Cool. It's a bit like what Kotlin did. Yet after some experience with Kotlin I found it is not perfect. "You have to be extra careful at boundaries" exactly, with Kotlin too.

The question mark is the best thing if you have not build your language with null safety to start with. Please look at Elm for a good example of what real null safety looks like.

With Go they had to change to do the right thing from the start. I really wonder why they didnt.