Since the book's publication, Google has made some changes to the Flutter API. In the following files, you must add setState(() {}); to the very end of the goToDetailPage method declaration: App0804.dart, App0805, and Figure0809Extensions.dart.
The Dart programming language has changed to include null safety. The quickest (and dirtiest) way to fix the book's code examples is to add a new line containing the words // @dart=2.9 to the top of each file.
