Swift: calling function from another viewcontroller makes objects nil

I have two simple ViewController: the first one has a IBOutlet linked to a UIImageView and a function that sets the image alpha channel. The second controller has a button that, when it's trigged, should call the function to set the alpha channel.

Here my code:

class FirstViewController: UIViewController {

     @IBOutlet imageView: UIImageView! 

     func changeAlphaChannel() {
         imageView.alpha = 0.5
     }
}

class SecondViewController: UIViewController {

     let firstController = FirstViewController()

     @IBAction func buttonPressed(sender: UIButton) {
         firstController.changeAlphaChannel()
     }
}

Calling changeAlphaChannel() inside FirstViewController works as aspected, but if I call it pressing the button, imageView becomes nil and the app crashes. How can I fix this? Thanks