     
     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()

        if userIsInTheMiddleOfTyping {
            guard let textCurrentlyInDisplay = display.text else { return }
            
            if digit == "." && (textCurrentlyInDisplay.range(of: ".") != nil) {
                return
            } else {
                let tmp = textCurrentlyInDisplay + digit
                display.text = tmp.setMaxLength(of: 8)
            }
            
        } else {
            if digit == "." {
                display.text = "0."
            } else {
                display.text = digit
            }

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