pendingError property Null safety

1. jrpn
2. controller
3. RunningController
4. pendingError property

RunningController class

1. Constructors
2. RunningController
3. Properties
4. argBase
5. currentDigitEntryState
6. gotoLineNumberKey
7. gsbOperation
8. gtoOperation
9. hashCode
10. keyboard
11. lastKey
12. minusOp
13. model
14. multOp
15. pause
16. pendingError
17. real
18. runtimeType
19. state
20. Methods
21. buttonDown
22. buttonUp
23. buttonWidgetDown
24. delayForShow
25. getArgsAndRun
26. getErrorNumber
27. handlePSE
28. newSelfTests
29. noSuchMethod
30. pasteToX
31. reset
32. resetAll
33. returnToParent
34. setArg
35. showCalculatorError
36. showMessage
37. singleStep
38. toString
39. Operators
40. operator ==