At the moment, the Execute Query Java action produces very ambiguous error messages under the following two circumstances I have identified:
The erorr message given is simply that a NullPointerException has occured in the microflow.
I propose that better error logging be introduced.
For circumstance 1, it could be flagged that no database column was found the specific property, and alert the developer to which property this.
This forum question is example of somebody facing this issue.
For circumstance 2, it could be flagged which column it was that had the NULL inside it.
I was only able to identify that these two issues were the root of my problem through many hours wasted, and trial and error. I now identify this ambiguous error with those circumstances, but other's should not have to waste their time trying to identify this too.