Added Mac target to iOS project in Xcode, now 'Error Starting Executable File'

I've decided to make a Mac OSX port of my iOS app, so based on a number of suggestions I've received I've simply added a new Cocoa target to my iOS project, and set up a series of 'libraries' (which encapsulate the core business logic of my app) which I have added to the OSX target. I have not added all of the UIViewController code to the OSX target - I will have to rewrite the UI code, for obvious reasons. The app is building fine (no compiler errors for mis开发者_Python百科sing libraries, etc.), but for some reason I get the following error message when I try to run the 'hello world' program, which I'd like to get working before I start coding in earnest. Here's the message:

Error Starting Executable 'MyExecutableMac'

No executable file found.
Use "file" or "exec-file" command.

As per this SO question, I made sure that the proper executable file was listed under the scheme build settings, so that it will actually run. However it actually appears that the .app file is not being packaged properly - the .app file is stil listed as "red" in the file list, meaning the file is missing. What might be causing this error?

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I had this problem in Xcode 4.2, I found that first doing a clean (Xcode > Product > Clean and option clicking Product > Clean Build Folder…) fixed it.