If you run the Tracer with all event types enabled, you should be able to see what stream is null. Also showing the error message from the PegaRULES log file with the stack trace (probably duplicated in the Tracer) will help.
I tried to generate the CSV file with the report definition as source, I got the results in the excel but I failed to the get the results from the join classes.
Example : If I have a report on X class and having the joins with Y and Z, then trying to generate the CSV then I am able to get the properties in X class only. pxPages data is not coming in the CSV files.