fKeyUser property Null safety

1. jrpn
2. jrpn15.main
3. CalculatorButtonWithUserMode
4. fKeyUser property

CalculatorButtonWithUserMode class

1. Constructors
2. CalculatorButtonWithUserMode
3. Properties
4. acceleratorKey
5. acceleratorLabel
6. bFactory
7. fKey
8. fKeyUser
9. fText
10. gKey
11. gText
12. gTextOffset
13. hashCode
14. height
15. innerBorder
16. innerBorderColor
17. key
18. keyTextOffset
19. keyTextStyle
20. lowerSurface
21. lowerSurfaceColor
22. lowerSurfaceColorPressed
23. outerBorder
24. outerBorderPressedScale
25. runtimeType
26. uKey
27. uKeyUser
28. upperSurface
29. upperSurfaceColor
30. upperSurfaceColorPressed
31. uText
32. width
33. Methods
34. createElement
35. createState
36. debugDescribeChildren
37. debugFillProperties
38. drawBlueText
39. drawGoldText
40. drawKeyboardAccelerator
41. drawText
42. drawWhiteText
43. noSuchMethod
44. paintForPainter
45. toDiagnosticsNode
46. toString
47. toStringDeep
48. toStringShallow
49. toStringShort
50. Operators
51. operator ==