Sorry for the trouble! 4.0 doesn't allow saving a project that contains errors. If you can please post or email us the
.spine project file, we can return it fixed up and also we'll make the next version of Spine fix the problem when a project loads, so others with the same project errors don't suffer the same problem.
Previously Spine would just save the project, but this can lead to nasty problems later. It's better now that users can see the problem and send us the project so we can fix things.