Error "The file 'MyApp.app' couldn't be opened because you don't have permission to view it" in Xcode

Hello, colleagues! I'm new to Swift and Xcode, and I've encountered a very strange problem. I'm using Xcode 6 Beta 4, writing a simple app in Swift. When I press the Run button (▶️), the build succeeds, Xcode says "Build Succeeded", but then an error dialog appears:\n\n"The file 'MyApp.app' couldn't be opened because you don't have permission to view it."\n\nHere is my simple code (file ViewController.swift):\n\n```swift\nimport UIKit\n\nclass ViewController: UIViewController {\n override func viewDidLoad() {\n super.viewDidLoad()\n let label = UILabel(frame: CGRect(x: 100, y: 100, width: 200, height: 50))\n label.text = "Hello, Swift!"\n label.textColor = UIColor.black\n self.view.addSubview(label)\n }\n}\n```\n\nNothing complicated, just a regular