Nil while caching images

Question

I've been able to solve the issue of caching images to improve scroll performance in my app. However nil is found when it tries to add it to cache. Also how can I add a placeholder image for images that failed to load or aren't available ?

let imageCache = NSCache<NSString, UIImage>()

    extension UIImageView {
    
        func downloadImage(from imgURL: String) -> URLSessionDataTask? {
            guard let url = URL(string: imgURL) else { return nil }
    
            // set initial image to nil so it doesn't use the image from a reused cell
            image = nil
    
            // check if the image is already in the cache
            if let imageToCache = imageCache.object(forKey: imgURL as NSString) {
                self.image = imageToCache
                return nil
            }
    
            // download the image asynchronously
            let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
                if let err = error {
                    print(err)
                    return
                }
    
                DispatchQueue.main.async {
                    // create UIImage
                    let imageToCache = UIImage(data: data!)
                    // add image to cache
                    imageCache.setObject(imageToCache!, forKey: imgURL as NSString)
                    self.image = imageToCache
                }
            }
            task.resume()
            return task
        }
    }