This started when a user reported that if you do not specify any parameters for the export handler, you get an NPE. I tracked it down to not specifying an "fl" parameter.
But in general I rearranged the error reporting in SortingResponseWriter.write so that immediately upon detecting a problem, the exception gets written to the output stream and then return immediately rather than save it up for the end. Preliminary version of the patch attached; it fixes the immediate problem.
Still to see is if it breaks any tests since the first error detected will be returned to the user rather than the last. I'll fix any tests that are sensitive to this and check in sometime this weekend.