I've had a lot of problems with the Pre-Print event. I have a script where I have to reverse all of my colors on the dark form I am creating to print mostly white. I couldn't get it to work in Pre-Print. Are you using a print button on the form? If you are try calling the script to make the fields hidden in the button click, before the script to execute the print.
Also, when you use the term "null", be sure that it is all lowercase. I don't think JavaScript will recognize "Null". It has to be "null".