ProductQueryPageHelper constructor Null safety

1. smooth_app
2. product_query_page_helper
3. ProductQueryPageHelper
4. ProductQueryPageHelper constructor

ProductQueryPageHelper class

1. Constructors
2. ProductQueryPageHelper
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. openBestChoice
9. toString
10. Operators
11. operator ==
12. Static methods
13. getDurationStringFromSeconds
14. getDurationStringFromTimestamp
15. getProductListLabel