register method Null safety

1. baguette
2. FirebaseNotificationsManager
3. FirebaseNotificationsManager
4. register method

FirebaseNotificationsManager class

1. Constructors
2. FirebaseNotificationsManager
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. addPlugin
8. cleanPlugins
9. getPlugin
10. noSuchMethod
11. register
12. removePlugin
13. toString
14. Operators
15. operator ==