I tried build project and i have this error.
2017-08-24 15:25:53.942 calculator[6278:756282] Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<calculator.ViewController 0x7fba6b509360> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key numbers.'
I have only this in ViewController...
@IBOutlet weak var label: UILabel!
@IBAction func numbers(_ sender: UIButton) {
label.text = label.text! + String(sender.tag-1)
}