ARQuicklook - Fatal error: Unexpectedly found nil while unwrapping an Optional value

Question

I am following this tutorial on how to make an AR Quicklook app. There are only a few steps which seems simple enough. However on the last step I am getting a fatal error because let variable is being force unwrapped. I have tried making it an optional instead but I get different errors like:

Optional chain has no effect, expression already produces 'URL?'

If I remove the optional I then get this warning on the next line down:

'URL?' is not convertible to 'QLPreviewItem'; did you mean to use 'as!' to force downcast?

If I force unwrap this line the app crashes. I can't work out how to get around this. I have even looked at the official video here and at around 14:30 minutes they also have the same code where they are force unwrapping that line.

@IBOutlet var collectionView: UICollectionView!
let models = ["A", "B", "C", "D", "E"]

var thumbnails = [UIImage]()
var thumbnailIndex = 0

override func viewDidLoad() {
    super.viewDidLoad()
    for model in models {
        if let thumbnail = UIImage(named: "\(model).jpg") {
            thumbnails.append(thumbnail)
        }
    }

    collectionView.dataSource = self
    collectionView.delegate = self
    collectionView.reloadData()
}

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return models.count
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "LibraryCell", for: indexPath) as? LibraryCollectionViewCell

    if let cell = cell {
        cell.modelThumbnail.image = thumbnails[indexPath.item]
        let title = models[indexPath.item]
        cell.modelTitle.text = title.capitalized
    }

    return cell!
}

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    thumbnailIndex = indexPath.item

    let previewController = QLPreviewController()
    previewController.dataSource = self
    previewController.delegate = self
    present(previewController, animated: true)
}

func numberOfPreviewItems(in controller: QLPreviewController) -> Int {
    return 1
}

func previewController(_ controller: QLPreviewController, previewItemAt index: Int) -> QLPreviewItem {
    let url = Bundle.main.url(forResource: models[thumbnailIndex], withExtension: "usdz")!
    return url as QLPreviewItem
}

Answer 1

This

let url = Bundle.main.url(forResource: models[thumbnailIndex], withExtension: "usdz")!

can only return nil if the item doesn't exist in main Bundle , or exists but target membership isn't checked , so verify that all these resources exist

A.usdz,B.usdz,C.usdz,D.usdz,E.usdz

the tutorial maker shows that here