MobileWikiAppReadingLists has a null webhost since a while back, is this an issue with the raw data?
hive (event)> select webhost from MobileWikiAppReadingLists where year=2019 and month=06 and webhost is not NULL limit 10;
OK
webhost
Time taken: 0.178 seconds
hive (event)> select webhost from MobileWikiAppReadingLists where year=2019 and month=05 and webhost is not NULL limit 10;
OK