Sounds like you may, like me, have accepted the suggestion by XCode 5.0 to migrate your icon files to a catalog.
When you do that, you need to remove the CFBundleIconFile (“Icon file”) parameter from your project’s .plist file. That’s the old way of doing this, and once you’ve migrated your icon files, obviously that parameter is incorrect.
Strange that XCode’s migration doesn’t fix this for you automatically. Would have been so very Apple to help you out there.