DropdownValues constructor Null safety

1. fact_browser
2. constants
3. DropdownValues
4. DropdownValues constructor

DropdownValues class

1. Constructors
2. DropdownValues
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Constants
12. categoryValues
13. correctnessValues
14. mediatypeValues