     
     private var displayValue: Double {
         get {
-            return Double(display.text ?? "") ?? Double.nan
+            return Double(display.text ?? Constants.emptyString) ?? Double.nan
         }
         set {
             let tmp = String(newValue).removeAfterPointIfZero()

    override func viewDidLoad() {
        // round the corners of the calculator on iPhones with the notch.
        if Device.allDevicesWithSensorHousing.contains(iPhoneModel) {
            cornerView.layer.cornerRadius = 35
            cornerView.layer.masksToBounds = true
        } 
    }

    }
}

extension ViewController {
    struct Constants {
        static let cornerRadius: CGFloat = 35.0
        static let decimalPoint: String = "."
        static let emptyString: String = ""
        static let maxStringLength: Int = 8
        static let pointAfterZero: String = "0."
    }
}
