title: Swift Sliders - fatal error: unexpectedly found nil while unwrapping an Optional value (lldb)
body: <p>I'm new to Swift and I was making a basic app that changes the text of a lable with the text in an inputbox and I wanted to add RGB sliders to change the color of the lable that will be changed but I got an errors soon as I try to move a slider.</p>
  
  <p><strong>fatal error: unexpectedly found nil while unwrapping an Optional value
  (lldb)</strong></p>
  
  <pre><code>@IBOutlet weak var OutputLabel: UILabel!
  @IBOutlet weak var InputField: UITextField!
  
  @IBOutlet weak var RedSlider: UISlider!
  @IBOutlet weak var BlueSlider: UISlider!
  @IBOutlet weak var GreenSlider: UISlider!
  
  @IBOutlet weak var RedLabel: UILabel!
  @IBOutlet weak var GreenLabel: UILabel!
  @IBOutlet weak var BlueLabel: UILabel!
  
  
  var RedColor : Float = 0
  var GreenColor : Float = 0
  var BlueColor : Float = 0
  
  override func viewDidLoad() {
      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 ChangeBtn(_ sender: Any) {
      OutputLabel.text = InputField.text
  }
  
  @IBAction func RedSliderAction(_ sender: UISlider) {
      changeColor()
  }
  
  @IBAction func GreenSliderAction(_ sender: UISlider) {
      changeColor()
  }
  
  @IBAction func BlueSliderAction(_ sender: UISlider) {
      changeColor()
  }
  
  
  func changeOutputLableColor() {
      OutputLabel.textColor = UIColor(red: CGFloat(RedColor), green: CGFloat(GreenColor), blue: CGFloat(BlueColor), alpha: 1.0)
      changeLabelNumbers()
  }
  
  func changeColor() {
      RedColor = RedSlider.value
      GreenColor = GreenSlider.value
      BlueColor = BlueSlider.value
      changeOutputLableColor()
  }
  
  func changeLabelNumbers() {
      let RoundedRed = String(format: "%0.0f", (RedColor * 255))
      let RoundedGreen = String(format: "%0.0f", (GreenColor * 255))
      let RoundedBlue = String(format: "%0.0f", (BlueColor * 255))
  
      RedLabel.text = "\(RoundedRed)"
      GreenLabel.text = "\(RoundedGreen)"
      BlueLabel.text = "\(RoundedBlue)"
  }
  </code></pre>
  
  <p>can someone help me figure it out?</p>