94Invalid use of the null

We are having difficulty with one of our experiments using MediaLab. We are repeatedly getting "94Invalid use of the null." We have re-made the experiment several times, checked through all of the questionnaire parameters (as per a google search of the error code recommended, [URL="http://www.empirisoft.com/Support/showthread.php?t=1253"][COLOR=#810081]http://www.empirisoft.com/Support/showthread.php?t=1253[/COLOR][/URL]). Notably the error changes places in our experiment (initially the error code was coming up after the last question on one questionnaire and then another questionnaire). However, when we removed both of these from the experiment, the error code came up at the end of the previously error-free questionnaire, then the last questionnaire in the revised experiment.
We are unsure how to proceed, as we have tried various edits and the location of the error message has changed repeatedly. Please let us know what the problem might be as soon as you can, as we are working under a deadline.

Hi Abby, are you able to get this error by running *any*single que file by itself, or is the running of the .exp file a necessary condition? Also—when “exactly” does the error occur (e.g., after the last item disappears but before the “tell the experimenter you are done” message)?