Issue with downloading youtube videos with swift - swift

Question

I am trying to download youtube video in my app and for that I used THIS and library included in that answer but my app is crashing with error :
fatal error: unexpectedly found nil while unwrapping an Optional value
Here is my swift code:
import UIKit
class ViewController: UIViewController {
let videoID = "hS5CfP8n_js"
var extractor = LBYouTubeExtractor()
override func viewDidLoad() {
super.viewDidLoad()
}
#IBAction func start(sender: AnyObject) {
var url: String = "https://www.youtube.com/watch?v=hS5CfP8n_js"
let testURL = NSURL(string:url)
if testURL != nil {
extractor = LBYouTubeExtractor(URL: testURL, quality: LBYouTubeVideoQualityLarge)
extractor.extractVideoURLWithCompletionBlock({(videoURL, error) in
if error == nil {
println(videoURL)
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
let data = NSData(contentsOfURL: videoURL)!
let pathToDocs = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] as! String
let fileName = "video_\(self.videoID).mp4"
let yourPath = pathToDocs.stringByAppendingPathComponent(fileName)
data.writeToFile(yourPath, atomically: true)
println("File \(fileName) successfully saved")
})
} else {
println("Failed extracting video URL using block due to error:\(error)")
}
})
}
}
}
my project is crashing at this line:
let data = NSData(contentsOfURL: videoURL)!
But I can get videoURL in console:
http%3A%2F%2Fr3---sn-tv0cgv5qc5oq-nu8e.googlevideo.com%2Fvideoplayback%3Fsver%3D3%26id%3D852e427cff27fe3b%26dur%3D55.681
... ty=small
I don't know what I am missing.
Here is my sample project.