Cannot determine the problem without the source. However, my guess is that either java.awt or javax.swing is used somewhere in it.

I lately had the same problem, and for me it was caused by running a later version of the jdk. It is fine in the application but the JavaScript conversion doesn’t seem to work. Try to reinstall Greenfoot, or download the zip and replace the jdk folder in your Greenfoot installation with the one from the zip. Finally, make sure to delete all the class files and use the recompile option from one of the drop down menus because sometimes Java ain’t recompile everything automatically and just show an error.