Change buttons text in container view

Question

I have a simple container view with button inside of this container view

@IBOutlet weak var containerView: UIView!
@IBOutlet weak var myButton: UIButton!

The class of container view in intarface builder is the initial "View Controller", but if i want to change text

viewDidLoad() {
    super.viewDidLoad()
    myButton.setTitle("aaa", for: .normal)
}

I have an error:

Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value How correctly use elements from container view?