I think NonNull should be enforced at two locations:

1. Here, required=True should be added:

 return DjangoListField(_type, required=True)

2. And here, NonNull() should be added:

 super(DjangoListField, self).__init__(List(NonNull(_type)), *args, **kwargs)

This is because:

1. When you retrieve the related Set of an object, you never get None : when no related items are returned, you get an empty Set.
2. You never, ever get a Set of None.

Current implementation causes issues when using third party tools that introspect the schema, like apollo-codegen. (One gets very annoying nested optionals of nested optionals that make no sense and require additional checking).

Hope that makes sense.
(I can do a PR)