sys property Null safety

1. weather
2. open_weather
3. OpenWeather
4. sys property

OpenWeather class

1. Constructors
2. OpenWeather
3. fromJson
4. Properties
5. base
6. clouds
7. cod
8. coord
9. dt
10. hashCode
11. id
12. main
13. name
14. rain
15. runtimeType
16. snow
17. sys
18. timezone
19. visibility
20. weather
21. wind
22. Methods
23. noSuchMethod
24. toJson
25. toString
26. Operators
27. operator ==