Auto Populate Forms Error

I have built several forms which successfully auto populate the fields based on information in other forms. I have used the instructions from Zoho. However, I am finding that often, probably every other time I try to use this script, I am given the error: Error modifying the page null. My past fixes have been to redevelop the form, but I am getting tired of duplicating my efforts and wasting my time. When I build a new form, I paste the exact same script and it works.

Any ideas on how to avoid this?