Returns nil if I scroll tableView fast

Question

Trying to load images in tableView asynchronously in (Xcode 9 and Swift 4) and seems I have a correct way but my code stops working if I scroll my tableView fast. So basically I had found nil error.
Here is my code:

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "cell", for: indexPath) as! CustomTableViewCell


    let feed = feeds[indexPath.row]

    cell.titleLabel.text = feed.title
    cell.pubDateLabel.text = feed.date
    cell.thumbnailImageView.image = nil

    if let image = cache.object(forKey: indexPath.row as AnyObject) as? UIImage {
        cell.thumbnailImageView?.image = image
    } else {

        let imageStringURL = feed.imageUrl

        guard let url = URL(string: imageStringURL) else { fatalError("there is no correct url") }

        URLSession.shared.downloadTask(with: url, completionHandler: { (url, response, error) in

            if let data = try? Data(contentsOf: url!) {
                DispatchQueue.main.async(execute: {
                    guard let image = UIImage(data: data) else { fatalError("can't create image") }

                    let updateCell = tableView.cellForRow(at: indexPath) as! CustomTableViewCell // fast scroll issue line

                    updateCell.thumbnailImageView.image = image
                    self.cache.setObject(image, forKey: indexPath.row as AnyObject)
                })
            }
        }).resume()
    }

    return cell
}

I have issue on the line:

let updateCell = tableView.cellForRow(at: indexPath) as! CustomTableViewCell

If I scroll down slowly everything works just fine and no mistakes appear.

Does anyone know where I've made a mistake?

Answer 1

This may happens if cell you are trying to get using tableView.cellForRow(at:) is not visible currently.
To avoid crash you can use optionals as:

let updateCell = tableView.cellForRow(at: indexPath) as? CustomTableViewCell // fast scroll issue line
updateCell?.thumbnailImageView.image = image

Keep everything as it is, I hope it should work without any errors.