addChild method Null safety

1. smooth_app
2. smooth_category_picker
3. SmoothCategory<T extends Comparable<T>>
4. addChild abstract method

SmoothCategory class

1. Constructors
2. SmoothCategory
3. Properties
4. hasChildren
5. hashCode
6. runtimeType
7. value
8. Methods
9. addChild
10. containsChildWithValue
11. findInDescendants
12. getChild
13. getChildren
14. getDescendants
15. getLabel
16. getParents
17. noSuchMethod
18. toString
19. Operators
20. operator ==