In my main data input in my Google App Maker app, I have relation data that's displayed in a list. I want the entries in that list to be able to be null until the user takes a specific action by clicking on a particular button, but at that point, I want the existence of values to be validated.
For the life of me, I couldn't figure out how to access iterate through the rows in the list and check the value of a specific field. But after a TON of trial and error, I figured it out -- see below.