fill property Null safety

1. jrpn
2. view.buttons
3. ButtonFactory
4. fill property

ButtonFactory class

1. Constructors
2. ButtonFactory
3. Properties
4. acceleratorTextStyle
5. context
6. controller
7. enterKeyTextOffset
8. fill
9. fKeyTextOffset
10. fTextOffset
11. fTextSmallLabelStyle
12. fTextStyle
13. gKeyTextOffset
14. gTextOffset
15. gTextStyle
16. gTextStyleForLJ
17. hashCode
18. height
19. innerBorder
20. keyTextOffset
21. keyTextStyle
22. lowerSurface
23. numColumns
24. numRows
25. onKeyTextOffset
26. outerBorder
27. padRight
28. runtimeType
29. screen
30. settings
31. shiftKeyTextStyle
32. specialButtonTextStyle
33. upperSurface
34. width
35. Methods
36. addUpperGoldLabels
37. buildButtons
38. buttonLayout
39. enterPos
40. noSuchMethod
41. toString
42. totalButtonHeight
43. Operators
44. operator ==