getOldestMessage method Null safety

1. baguette
2. MessengerManager
3. MessengerManager
4. getOldestMessage method

MessengerManager class

1. Constructors
2. MessengerManager
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. addPlugin
8. cleanPlugins
9. getConversation
10. getConversations
11. getMessage
12. getMessages
13. getOldestMessage
14. getPlugin
15. loadConversations
16. loadMessages
17. noSuchMethod
18. removePlugin
19. toString
20. Operators
21. operator ==