title: fatal error: unexpectedly found nil while unwrapping an Optional value while trying to change UILabel text
body: <p>I'm making a simple temperature converter for a mobile apps class using Swift.
  We're sort of breezing over swift so I don't have a lot of experience with it. I had a lot of problems getting it to compile but I eventually got everything all nice and neat, no warning or errors of any sort, and I'm happy with my MVC style code. The only problem is that when I enter a temp and click the Convert button, my log checkpoints prove that I'm getting he correct double value for conversion, and I caste it as a String, but when I try to change the outputText label I get the fatal error: unexpectedly found nil while unwrapping an Optional value. I was hoping it's something very simple that a second pair of eyes could catch.
  My app looks like this...</p>
  
  <p><a href="https://i.stack.imgur.com/7xeuI.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/7xeuI.png" alt="App view"></a></p>
  
  <p>My ViewController is...</p>
  
  <pre><code>import UIKit
  
  class ViewController: UIViewController
  {
  
      @IBOutlet weak var tempInput: UITextField!
      @IBOutlet weak var tempOutput: UILabel!
      @IBOutlet weak var convertMethod: UISwitch!
  
      override func viewDidLoad()
      {
          print("view load confirmed");
          super.viewDidLoad()
          // Do any additional setup after loading the view, typically from a nib.
      }
  
      override func didReceiveMemoryWarning()
      {
          super.didReceiveMemoryWarning()
          // Dispose of any resources that can be recreated.
      }
  
      @IBAction func convert(sender: AnyObject!)
      {
          print ("button press confirmed");
  
          let t = Double(tempInput.text!)
          let m = convertMethod.on
          let result = convertModel(temp:t!,method:m)
          print("result has been set to object variable created as stated above");
  
          let afterConvert = result.convertTemp()
          print("the convertTemp method was called and the result was stored as the double: \(afterConvert)");
  
          let finalForm = String(afterConvert)
          print("converted the double into the string: \(finalForm)");
  
          tempOutput.text = finalForm
          //^^^^^^^ this is the line that fails
      }
  }
  </code></pre>
  
  <p>and my convertModel class (i know, the class should be capitalized, by the time I noticed and tried to change it, it broke everything) is...</p>
  
  <pre><code>import Foundation
  class convertModel
  {
      var temp:Double?
      var method:Bool?
  
      init(temp:Double,method:Bool)
      {
          self.temp = temp
          self.method = method
          print("object of type convertModel created with \(temp) as temp and \(method) as method");
      }
  
      func convertTemp()-&gt;Double
      {
          print("convertTemp method entered");
  
          if method == true
          {
              print("if conditional entered");
              print("conversion completed, exiting convertTemp method");
  
              return (temp! * (9/5) + 32)
          }
          else
          {
              print("else conditional entered");
              print("conversion completed, exiting convertTemp method");
              return (temp! - 32) * (5/9)
          }
      }
  }
  </code></pre>
  
  <p>Any help is greatly appreciated. But please bear in mind that this is very Swift 101, so any overly complicated solutions don't help greatly. Thanks.</p>