toJson method Null safety

1. flutter_json_widgets
2. form_data
3. FormField
4. toJson method

FormField class

1. Constructors
2. bool
3. color
4. dateTime
5. double
6. duration
7. fromJson
8. int
9. map
10. num
11. object
12. string
13. Properties
14. copyWith
15. description
16. hashCode
17. key
18. runtimeType
19. value
20. Methods
21. map
22. mapOrNull
23. maybeMap
24. maybeWhen
25. noSuchMethod
26. toJson
27. toString
28. when
29. whenOrNull
30. Operators
31. operator ==