StackFit constructor Null safety

1. flutter_json_widgets
2. enums
3. StackFit
4. StackFit const constructor

StackFit enum

1. Constructors
2. StackFit
3. Values
4. loose
5. expand
6. passthrough
7. Properties
8. hashCode
9. index
10. runtimeType
11. Methods
12. noSuchMethod
13. toString
14. Operators
15. operator ==
16. Constants
17. values