CommandServiceBase constructor Null safety

1. spine_client
2. command_service.pbserver
3. CommandServiceBase
4. CommandServiceBase constructor

CommandServiceBase class

1. Constructors
2. CommandServiceBase
3. Properties
4. $json
5. $messageJson
6. hashCode
7. runtimeType
8. Methods
9. createRequest
10. handleCall
11. noSuchMethod
12. post
13. toString
14. Operators
15. operator ==