handleCHS abstract method Null safety

1. jrpn
2. controller.states
3. ActiveState
4. handleCHS abstract method

ActiveState class

1. Constructors
2. ActiveState
3. Properties
4. controller
5. hashCode
6. model
7. runtimeType
8. stackLiftEnabled
9. Methods
10. abort
11. buttonDown
12. buttonUp
13. changeState
14. getCalculation
15. gosubDoneState
16. gosubEntryDone
17. handleBackspace
18. handleBST
19. handleCHS
20. handleClearPrefix
21. handleClearProgram
22. handleDecimalPoint
23. handleEEX
24. handleGotoDot
25. handleLetterLabel
26. handleNumberKey
27. handleOnOff
28. handlePR
29. handlePSE
30. handleRunStop
31. handleShift
32. handleShow
33. handleShowImaginary
34. handleShowMem
35. handleShowStatus
36. handleSST
37. liftStackIfEnabled
38. noSuchMethod
39. onArgComplete
40. onChangedTo
41. terminateDigitEntry
42. toString
43. unreachable
44. Operators
45. operator ==