Deprecated Topic Handling

[svzi]

When I subscribe or unsubscribe to/from a topic, I always see this message on iOS in my console:

2019-08-20 22:01:00.901017+0200 ProjectName[6558:6351179] 6.3.0 - [Firebase/Messaging][I-FCM002024] Format '/topics/de_beta' is deprecated. Only 'de_beta' should be used in subscribeToTopic.

This is only a deprecation warning, so I hope this is not the reason for this issue:
#104