Workable Flutter_book code

[DenisNaN]

Just started my journey in mobile development. I really like the book, but the code in it does not work because of null safety. Therefore, I corrected the code due to my abilities. Don't scold too much. But for those who are starting to learn Flutter, it's better to face what works.

https://github.com/DenisNaN/Flutterbook

[aemxn]

@DenisNaN thanks for this. I was wondering how to make it work with latest Dart. please don't delete your repository just yet. I'm still learning.