returnStackSize property Null safety

1. jrpn
2. model
3. Model<OT extends ProgramOperation>
4. returnStackSize property

Model class

1. Constructors
2. Model
3. Properties
4. captureDebugLog
5. cFlag
6. debugLog
7. display
8. displayDisabled
9. displayLeadingZeros
10. displayMode
11. doubleWordStatus
12. errorBlink
13. floatOverflow
14. gFlag
15. hashCode
16. integerSignMode
17. internalSnapshot
18. is15C
19. isComplexMode
20. isFloatMode
21. lastX
22. lastXC
23. lastXImaginary
24. logicalKeys
25. maxInt
26. memory
27. minInt
28. modelName
29. needsSave
30. onIsPressed
31. popSetResultX
32. popSetResultXC
33. popSetResultXF
34. popSetResultXI
35. prgmFlag
36. program
37. registerNumberBase
38. resultX
39. resultXC
40. resultXF
41. resultXI
42. returnStackSize
43. runtimeType
44. settings
45. shift
46. signMask
47. signMode
48. t
49. trigMode
50. userMode
51. wordMask
52. wordSize
53. x
54. xC
55. xF
56. xI
57. xImaginary
58. xPreserveCLX
59. xRealF
60. y
61. yC
62. yF
63. yI
64. z
65. zC
66. zF
67. Methods
68. addProgramTraceToSnapshot
69. addStuffToSnapshot
70. chsX
71. clx
72. compare
73. decodeJson
74. formatValue
75. getFlag
76. getStackByIndex
77. getStackByIndexC
78. initializeFromJsonOrUri
79. isZero
80. newProgramInstruction
81. noSuchMethod
82. optimizeInternalSnapshot
83. popStack
84. pushStack
85. readFromPersistentStorage
86. reset
87. resetErrorBlink
88. resetFromPersistentStorage
89. rotateStackDown
90. rotateStackUp
91. selfTestContents
92. setFlag
93. setupComplex
94. setXYZT
95. setYZT
96. swapXY
97. toJson
98. toString
99. tryParseValue
100. writeToPersistentStorage
101. Operators
102. operator ==