NavigatorObserver constructor Null safety

1. Flutter
2. widgets
3. NavigatorObserver
4. NavigatorObserver constructor

NavigatorObserver class

1. Constructors
2. NavigatorObserver
3. Properties
4. hashCode
5. navigator
6. runtimeType
7. Methods
8. didPop
9. didPush
10. didRemove
11. didReplace
12. didStartUserGesture
13. didStopUserGesture
14. noSuchMethod
15. toString
16. Operators
17. operator ==