The Apple engineers are kinda busy elsewhere until next week, so I doubt you'll see them responding in the next few days, though I'd expect their presence to normalize after that.
Clearly there's a bug here, and I haven't seen anyone suggest a workaround, so it may be unavoidable until a new Xcode beta is issued, which will also be after WWDC. You should file a bug report, including your project, to make sure your issue gets looked into.
Since you got the error after creating a new project in Xcode 7, you could try deleting it and re-creating it in Xcode 6, then opening it in Xcode 7. When I tried this, Xcode 7 offered to make some changes to the storyboard which I declined, and then I set the scene's "Opens in" setting to 6.3 explicitly. Doing something along that line might allow you to proceed for now.