ScreenKey constructor Null safety

1. baguette
2. ScreenKey
3. ScreenKey
4. ScreenKey constructor

ScreenKey class

1. Constructors
2. ScreenKey
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Constants
12. ENVIRONMENT_SCREEN
13. PREVIOUS_SCREEN
14. ROOT_SCREEN
15. TAG