I am still getting the error for "Null Undefined" where the server validations should be.
This time, I made sure to denote what is happening.
I found that the Form Builder beta works fine before installing the HTML Editor 3 beta.
I ran the test a few times without the the new HTML Editor 3 and all the validations work fine.
Only after installing the HTML Editor 3 do I get that issue - which is strange because some of the forms I was creating didn't even have an instance of HTML Editor.
So my guess is that with code in Form Builder that gets updated when HTML Editor 3 is installed, it must also be causing a conflict with other attributes in the Form Builder extension.
Other than that, I can confirm that previews now work great inside the dialog box when you add an HTML Editor instance.