email property Null safety

1. next_photo
2. features
3. UserModel
4. email property

UserModel class

1. Constructors
2. UserModel
3. fromJson
4. Properties
5. address
6. company
7. copyWith
8. email
9. hashCode
10. id
11. name
12. phone
13. runtimeType
14. username
15. website
16. Methods
17. noSuchMethod
18. toJson
19. toString
20. Operators
21. operator ==