Xcode Swift app keeps crashing and I am not sure why?

import UIKit

class ViewController: UIViewController {
    // Instance Variable (Property)
    // This is actuall an optional (even tho it does not look like one.
    // The "!" below means that we do not have to put it after "display" everytime we write it. Implicitly Unwrapped Optional.
    @IBOutlet weak var display: UILabel!

    var userIsInTheMiddleOfTypingANumber = false


    @IBAction func appendDigit(sender: UIButton) {
        // Local variable aming the digit equal the number on the button.
        // The ! will unwrap the optionals, ie. the button values.
        let digit = sender.currentTitle!


        if userIsInTheMiddleOfTypingANumber {
            // display.text is an optional that can be a string. It must be unwrapped in order for it to be added to another string.
            display.text = display.text! + digit
        } else {
            display.text = digit
            userIsInTheMiddleOfTypingANumber = true
        }
    }



    @IBAction func operate(sender: UIButton) {
        let operation = sender.currentTitle!

        // Anytimee an operator is pressed.
        if userIsInTheMiddleOfTypingANumber{
            enter()
        }

        // Switch absolutely requires everycase is covered.
        switch operation {
        case "×": performOperation({ (op1: Double, op2: Double) -> Double in return op1 * op2
            })
//            case "÷":
//            case "+":
//            case "−":
            default: break
        }
    }



    func performOperation(operation: (Double, Double) -> Double) {
        if operandStack.count >= 2 {
            displayValue = operation(operandStack.removeLast(), operandStack.removeLast())
            enter()
        }
    }



    // Initializer.
    // The <> indicate what is in the array. When a button is pressed it is stored in an Array (see console when app is running).
    var operandStack = Array<Double>()

    @IBAction func enter() {
        userIsInTheMiddleOfTypingANumber = false
        operandStack.append(displayValue)
        println("operandStack = \(operandStack)")
    }

    // Computed Properties
    var displayValue: Double {
        get {
          return NSNumberFormatter().numberFromString(display.text!)!.doubleValue
        }
        set {
            // Converting "newValue" into a String.
            display.text = "\(newValue)"
            userIsInTheMiddleOfTypingANumber = false
        }
    }

}

   if operandStack.count >= 2 {