This particular instance is when using the HTML Editor interface. The only time I applied it through Form Builder, I didn't customize the toolbar so I am not sure if that would create the same problem.
I've tried numerous times to get the error message you mentioned to appear by inserting a new form with the Form Builder Wizard and then editing the applied HTML Editor as well have with direct inserts with HTML Editor itself. I haven't had any luck. Do you have other extensions installed with this beta build? Are there more detailed steps you can provide?