getDouble method Null safety

1. baguette
2. PreferencesManager
3. PreferencesManager
4. getDouble method

PreferencesManager class

1. Constructors
2. PreferencesManager
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. addPlugin
8. cleanPlugins
9. getDouble
10. getInt
11. getPlugin
12. getString
13. noSuchMethod
14. remove
15. removePlugin
16. setDouble
17. setInt
18. setString
19. toString
20. Operators
21. operator ==