Xcode 4.3 - MainStoryboard_iPhone.storyboard could not be opened

I get an error in Xcode 4.3:

The document "MainStoryboard_iPhone.storyboard" could not be opened. The operation couldn’t be completed. (com.apple.InterfaceBuilder error -1.)

I can't build the app either. I think it could be because I made a few changes to the storyboard in Xcode 4.5-DP1 instead of Xcode 4.3. I saved the project in Xcode 4.5 and tried exiting out of & restarting Xcode 4.3 to see if that would help, but I'm still having trouble.

Try the Apple developer forums, I'm not sure you'll find many people on a VB forum that are familiar with XCode :p