NERoomCustomMessage constructor Null safety

1. yunxin_roomkit
2. roomkit
3. NERoomCustomMessage
4. NERoomCustomMessage constructor

NERoomCustomMessage class

1. Constructors
2. NERoomCustomMessage
3. Properties
4. attachStr
5. fromAccount
6. fromNick
7. hashCode
8. messageType
9. runtimeType
10. time
11. toAccounts
12. Methods
13. noSuchMethod
14. toString
15. Operators
16. operator ==