handleBackspace abstract method Null safety

1. jrpn
2. controller.states
3. LimitedState
4. handleBackspace abstract method

LimitedState class

1. Constructors
2. LimitedState
3. Properties
4. controller
5. hashCode
6. model
7. runtimeType
8. Methods
9. abort
10. buttonDown
11. buttonUp
12. changeState
13. getCalculation
14. gosubDoneState
15. gosubEntryDone
16. handleBackspace
17. handleBST
18. handleClearPrefix
19. handleClearProgram
20. handleDecimalPoint
21. handleGotoDot
22. handleOnOff
23. handlePR
24. handleShift
25. handleShowImaginary
26. handleShowMem
27. handleShowStatus
28. handleSST
29. noSuchMethod
30. onArgComplete
31. onChangedTo
32. terminateDigitEntry
33. toString
34. unreachable
35. Operators
36. operator ==