Just a heads up, I was getting the same error as this, as well.
To stop it from happening, I noticed when I deinstalled and then reinstalled the extension, that seemed to clear that error.
But then I started getting a different issue. Similar to an error, I experienced in an earlier build of Form Builder, I noticed that when creating the registration page, I would get the word "undefined" where the server validations we're supposed to go. And just to confirm, I am using version 2.0.1 which was intended to fix the "undefined" error but I still get it.
I poked around but I am not sure how to fix that one.