I was stumped when IntelliJ consistently errored out when choosing Form main() from the insert menu, with an error message saying something about an invalid root component.
It turns out it means that the JPanel that is the main (root) component of your Form needs to have a field bound to it, and by default it doesn’t do this.
Simply select the JPanel in the form designer, and give it a name (field name) in the property pane.
After this it will generate the main() just fine.