viewModel property Null safety

1. empire
2. empire_state
3. EmpireWidget<T extends EmpireViewModel>
4. viewModel property

EmpireWidget class

1. Constructors
2. EmpireWidget
3. Properties
4. debugPrintStateChanges
5. hashCode
6. key
7. runtimeType
8. viewModel
9. Methods
10. createElement
11. createEmpire
12. createState
13. debugDescribeChildren
14. debugFillProperties
15. noSuchMethod
16. toDiagnosticsNode
17. toString
18. toStringDeep
19. toStringShallow
20. toStringShort
21. Operators
22. operator ==