getFirebaseMessaging method Null safety

1. baguette
2. FirebaseManager
3. FirebaseManager
4. getFirebaseMessaging method

FirebaseManager class

1. Constructors
2. FirebaseManager
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. addPlugin
8. cleanPlugins
9. crash
10. ensureInitialization
11. getFirebaseAnalytics
12. getFirebaseApp
13. getFirebaseMessaging
14. getFirebaseStore
15. getPlugin
16. noSuchMethod
17. removePlugin
18. toString
19. Operators
20. operator ==
21. Constants
22. TAG