FlexParentData constructor Null safety

1. Flutter
2. rendering
3. FlexParentData
4. FlexParentData constructor

FlexParentData class

1. Constructors
2. FlexParentData
3. Properties
4. fit
5. flex
6. hashCode
7. nextSibling
8. offset
9. previousSibling
10. runtimeType
11. Methods
12. detach
13. noSuchMethod
14. toString
15. Operators
16. operator ==