If you define a method that returns a nullable Map:

@HostApi()
abstract class PushHostApi {
  Map<String?, Object?>? getNotification();
}

This generates some code, which has 1 invalid line:

return (replyMap['result'] as Map<Object?, Object?>?).cast<String?, Object?>();

There is a quick fix: add ? to the line, making it return (replyMap['result'] as Map<Object?, Object?>?)?.cast<String?, Object?>();

Looks like this was one edge case that was not supported as part of 2.0.0