SubscriptionServiceBase constructor Null safety

1. spine_client
2. subscription_service.pbserver
3. SubscriptionServiceBase
4. SubscriptionServiceBase constructor

SubscriptionServiceBase class

1. Constructors
2. SubscriptionServiceBase
3. Properties
4. $json
5. $messageJson
6. hashCode
7. runtimeType
8. Methods
9. activate
10. cancel
11. createRequest
12. handleCall
13. noSuchMethod
14. subscribe
15. toString
16. Operators
17. operator ==