I wanted to try adding a custom non-palette component (JButton subclass) to a dialog. Everything went smoothly until I tried to preview the form, when all I got was the error popup.
Killed and restarted #5594, and created a new project and a form with a standard JTextField dragged onto it. Tried to preview it and got the error popup again - no preview.
Looks like preview is plain broken in #5594...?