I am trying out Tableau prep and last week I have published a flow to my server. Now today I am trying and the query runs fine but when in the publishing phase I get the following error. System Error: java.lang.NullPointerException. I upgraded to 2018.2.1 today so wondering if that is part of the cause? I also ensured to filter out any null values I might have had in my data.
This seems to be cause by a Defect with ID 800231.
The fix will be implemented in a future Tableau Prep release.
Try to use one of the workaround options documented in the article below.
Hope this help