Thread 1: Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value

I am a beginner to swift language and with the help of apple book, I was typing this code on switching colours using buttons. then in this line: if redswitch.isOn { it is showing this fatal error. What to do I have no idea. Please help me out.

Here's the code: import UIKit

class ViewController: UIViewController {

@IBOutlet weak var colourView: UIView!
@IBOutlet weak var redswitch: UISwitch!
@IBOutlet weak var greenswitch: UISwitch!
@IBOutlet weak var blueswitch: UISwitch!


override func viewDidLoad() {
    super.viewDidLoad()
    
    
    updateColor()
    // Do any additional setup after loading the view.
}
@IBAction func switchChanged(_sender: UISwitch){
    updateColor()
}
func updateColor() {
    var red: CGFloat = 0
    var green: CGFloat = 0
    var blue: CGFloat = 0

    if redswitch.isOn {
        red = 1
    }
    if greenswitch.isOn {
        green = 1
    }
    if blueswitch.isOn {
        blue = 1
    }
    let color = UIColor(red: red, green: green,
       blue: blue, alpha: 1)
    colourView.backgroundColor = color
}

}