fatal error: unexpectedly found nil while unwrapping an Optional value

  override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        let baseURL = NSURL(string: "https://api.forecast.io/forecast/\(apiKey)/")
        let forecastURL = NSURL(string: "48.189250,16.390753", relativeToURL: baseURL)
        let sharedSession = NSURLSession.sharedSession()
        let downloadTask: NSURLSessionDownloadTask = sharedSession.downloadTaskWithURL(forecastURL!, completionHandler: {(location: NSURL!, response: NSURLResponse!, error: NSError!) -> Void in

            if error == nil {
                let dataObject = NSData(contentsOfURL: location)
                let weatherDictionary: NSDictionary = NSJSONSerialization.JSONObjectWithData(dataObject!, options: nil, error: nil) as NSDictionary
                let currentWeather = Current(weatherDictionary: weatherDictionary)
                println(currentWeather.temperature)
            }
        })
        downloadTask.resume()
    }

struct Current {

    var currentTime: Int
    var temperature: Int
    var humidity: Double
    var precipProbability: Double
    var summary: String
    var icon: String

    init(weatherDictionary: NSDictionary) {

        let currentWeather = weatherDictionary["current"] as NSDictionary

        currentTime = currentWeather["time"] as Int
        temperature = currentWeather["temperature"] as Int
        humidity = currentWeather["humidity"] as Double
        precipProbability = currentWeather["precipProbability"] as Double
        summary = weatherDictionary["summary"] as String
        icon = weatherDictionary["icon"] as String

    }

}