XCode won't open a Project

Today I was trying to work on a project in XCode (which I'm very unfamiliar with) and it refused to open the project giving the following error:

Internal Error  
File:  /SourceCache/DevToolsBase/.../XCPlatformSpecification.m  
Line:  438  
Object:    
Method:  loadAllPlatforms  
The Mac OS X platform is missing - cannot set a default platform.

After some googling around someone mentioned deleting a cache file, well, I could find no such cache file. However, I could start XCode and in the file menu was the option to "Empty Caches". Doing that fixed the problem. So just a short FYI.