Update code for null-safety

[warcayac]

Sound null safety is available in Dart 2.12 and Flutter 2.

This issue could be a big problem for a beginner using the latest version of Dart.

[nyck33]

flutter migrate on flutter.dev worked for me but there are null errors related to things like Focus Scope in Ch.6 that if you comment out, go away. Or just go through tutorial on Focus, Focus Node from flutter.dev and learn to use ti properly. I just commented it out for now.