Sometimes the page elements still not load, it will return "null".
I think we can set the elements delay until find the elements when page load. Or if return "null", only run the "null" pages again until the return not “null”.
In some cases always is null because the element not exist it's the same you up the delay, with retry only get data trash.
If you know not have all data scraped, do two passes, the second only with urls not get data and add more delay...