toJson method Null safety

1. baguette
2. Proficiency
3. Proficiency
4. toJson method

Proficiency class

1. Constructors
2. Proficiency
3. fromJson
4. Properties
5. hashCode
6. level
7. name
8. runtimeType
9. Methods
10. noSuchMethod
11. toJson
12. toString
13. Operators
14. operator ==