Ok, so I have a Nintex form which uses a Single Line Text box as a parameter to fill a SQL Request lookup.
This works well. You enter the text into the search box, press tab and the dropdown gets populated with your result.
Then, based on the users choice in the dropdown, I had one or more extra SQL Requests going and getting more data relevant to the result. IE for example choosing a user and then going off and getting their office, department, manager etc.
However on doing this I would get the following error on loading the form.
Unable to get property 'toString' of undefined or null reference.
After a fair bit of trial and error, I stumbled upon the following simple workaround.
Rather than using the SQL Dropdown to pass the parameter into the SQL lookup, I created a calculated field looking at the SQL Dropdown field and passed that into the SQL lookup instead. No more error.