message property Null safety

1. diginote
2. message_item_content
3. MessageItemContent
4. message property

MessageItemContent class

1. Constructors
2. MessageItemContent
3. Properties
4. hashCode
5. height
6. key
7. message
8. runtimeType
9. selected
10. width
11. Methods
12. build
13. createElement
14. debugDescribeChildren
15. debugFillProperties
16. noSuchMethod
17. toDiagnosticsNode
18. toString
19. toStringDeep
20. toStringShallow
21. toStringShort
22. Operators
23. operator ==