Adding fields to a existing document in Firestore


I have a collection of docs and now I have decided to add several fields to each one of them. In order to manage it better than in Firestore web, I have created a list in a screen populating all data from my documents like this:

The empty fields are the one to write a new value and then save data with the icon in the right. So far so good. I configure the button action like this:

My problem comes because these fields don’t exist yet into the document. And when I try click in the icon to save the data and add those fields I get an error message:

How do I have to do it in order to create these new fields and their values in existing documents?

Thank you.