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

import UIKit

class ViewController: UIViewController {

    private let imageView: UIImageView = {
        let imageView = UIImageView()
        imageView.contentMode = .scaleAspectFill
        imageView.backgroundColor = .white
        return imageView
    }()
    
    private let button: UIButton = {
        let button = UIButton()
        button.backgroundColor = .white
        button.setTitle("Button", for: .normal)
        button.setTitleColor(.orange, for: .normal)
        return button
    }()
    
    let colors: [UIColor] = [
        .systemRed,.systemBlue,.systemGreen,.systemOrange,.systemGray6]
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .systemGray
        // Do any additional setup after loading the view.
        view.addSubview(imageView)
        imageView.frame = CGRect(x: 50, y: 150, width: 300, height: 300)
        imageView.center = view.center
        
        view.addSubview(button)
        getRandomPhoto()
        button.addTarget(self, action: #selector(didTapButton), for: .touchUpInside)

}
    
    @objc func  didTapButton() {
        getRandomPhoto()
        
        view.backgroundColor = colors.randomElement()
    }
    
    override func viewWillLayoutSubviews() {
        super.viewDidLayoutSubviews()
        button.frame = CGRect(x: 30, y: view.frame.size.height-150-view.safeAreaInsets.bottom, width: view.frame.size.width-60, height: 55)

    }
    
    func getRandomPhoto() {
        let urlString = "https://source.unsplash.com/random/600×600"
        let url = URL(string: urlString)!
        guard  let data = try? Data(contentsOf: url) else {
            return
        }
        imageView.image = UIImage(data: data)
    }
}

Xcode初心者です。
コードを打ち終わりシュミレーションをしようとすると強制終了になり、let url = URL(string: urlString)!というコードの横にThread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional valueというエラーが表示されます。
解決方法を教えてください。