ChatroomMessageBuilder constructor Null safety

1. nim_core
2. nim_core
3. ChatroomMessageBuilder
4. ChatroomMessageBuilder constructor

ChatroomMessageBuilder class

1. Constructors
2. ChatroomMessageBuilder
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Static methods
12. createChatroomAudioMessage
13. createChatroomCustomMessage
14. createChatroomFileMessage
15. createChatroomImageMessage
16. createChatroomLocationMessage
17. createChatroomRobotMessage
18. createChatroomTextMessage
19. createChatroomTipMessage
20. createChatroomVideoMessage