The main issue with non-nullable types in OOP, in my mind, seems to be defaults. Quite simply, when you don't initialize an primitive, you get a 0, and when you don't initialize an object, you get a null. Not all fields must be initialized by the time the constructor ends, so some kind of valid default exists.

Enforcing non-nullability would involve somehow enforcing that the variable is always valid before use some other way, like how "final" is enforced in the constructor, but it should be possible.

Unless you make it default, though, it won't be as "elegant" as the haskell Maybe, and doing so would net you a very very different language.