callback property Null safety

1. baguette
2. NavigationBar
3. NavigationBar
4. callback property

NavigationBar class

1. Constructors
2. NavigationBar
3. Properties
4. actions
5. callback
6. hashCode
7. index
8. key
9. preferredSize
10. runtimeType
11. Methods
12. createElement
13. createState
14. debugDescribeChildren
15. debugFillProperties
16. noSuchMethod
17. toDiagnosticsNode
18. toString
19. toStringDeep
20. toStringShallow
21. toStringShort
22. Operators
23. operator ==