     //MARK: IBAction(s)
     
     @IBAction func touchDigit(_ sender: UIButton) {
-        let digit = sender.currentTitle!
+        guard let digit = sender.currentTitle else { return }
         
         if userIsInTheMiddleOfTyping {
-            let textCurrentlyInDisplay = display.text!
+            guard let textCurrentlyInDisplay = display.text else { return }
             
             if digit == "." && (textCurrentlyInDisplay.range(of: ".") != nil) {
                 return

    //MARK: UIVIew Delegate
    
    override func viewDidLoad() {
        
        // round the corners of the calculator on iPhone X
        if iPhoneModel == .iPhoneX || iPhoneModel == .simulator(.iPhoneX){
            cornerView.layer.cornerRadius = 35
            cornerView.layer.masksToBounds = true
        } 

    }
    
    @IBAction func performOperation(_ sender: UIButton) {
        if userIsInTheMiddleOfTyping {
            brain.setOperand(displayValue)
            userIsInTheMiddleOfTyping = false
