PreferredSizeWidget constructor Null safety

1. Flutter
2. widgets
3. PreferredSizeWidget
4. PreferredSizeWidget constructor

PreferredSizeWidget class

1. Constructors
2. PreferredSizeWidget
3. Properties
4. hashCode
5. key
6. preferredSize
7. runtimeType
8. Methods
9. createElement
10. debugDescribeChildren
11. debugFillProperties
12. noSuchMethod
13. toDiagnosticsNode
14. toString
15. toStringDeep
16. toStringShallow
17. toStringShort
18. Operators
19. operator ==