ProcessWire is version 2.4.0.
FormBuilder is version 0.2.2.
The error occurs in attempting to preview a form ("contact") that had previously been created. The form has 4 fields: first_name (text), last_name (text), email_address (email) and phone_number (text). When I click on the Preview tab, it displays the current page inside the iframe with an additional error message "Unknown Form - 0" at the top.
I am not using any code at all, so I don't know what you're referring to with "$id".
If I attempt to embed it into a page with "form-builder/contact", the iframe errors on display because the src is pointing to an invalid url ("[site].comcontact"). We tried creating a new form from scratch ("contactinfo"), but it does exactly the same thing in preview and embedding.