title: Geting a fatal "error: unexpectedly found nil while unwrapping an Optional value" when chaging value of label on button press
body: <p>I keep getting nil value when I press button.</p>
  
  <pre><code>import UIKit
  
  class ViewController: UIViewController {
   //MARK: Properties
  
   @IBOutlet weak var txt_vrednost: UITextField!
   @IBOutlet weak var btn_dodaj: UIButton!
   @IBOutlet weak var lbl_stanje: UILabel!
   var novcanik = 0.0
  
   //MARK: Acttion
  
   @IBAction func btn_dodaj_pressed(_ sender: Any) {
  
       let values = Double(txt_vrednost.text!)
       novcanik = novcanik + values!
       lbl_stanje.text=String(novcanik)
  
   }
  
   override func viewDidLoad() {
       super.viewDidLoad()
  
       // Do any additional setup after loading the view, typically from a nib.
    }
  }
  </code></pre>