CoreLocation: fatal error: unexpectedly found nil while unwrapping an Optional value

import UIKit
import CoreLocation

class ViewController: UIViewController, CLLocationManagerDelegate {

    var manager: CLLocationManager!
    var currentLocation: CLLocation!

    private let apiKey = "#################"

    @IBOutlet weak var iconView: UIImageView!
    @IBOutlet weak var currentTimeLabel: UILabel!
    @IBOutlet weak var temperatureLabel: UILabel!
    @IBOutlet weak var humidityLabel: UILabel!
    @IBOutlet weak var precipitationLabel: UILabel!
    @IBOutlet weak var summaryLabel: UILabel!
    @IBOutlet weak var refreshButton: UIButton!
    @IBOutlet weak var refreshActivityIndicator: UIActivityIndicatorView!


    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        refreshActivityIndicator.hidden = true

        manager = CLLocationManager()
        manager.delegate = self
        manager.desiredAccuracy = kCLLocationAccuracyBest
        manager.requestAlwaysAuthorization()


    }

    func locationManager(manager: CLLocationManager!, didChangeAuthorizationStatus status: CLAuthorizationStatus) {
        self.manager.stopUpdatingLocation()
        if status == .Authorized || status == .AuthorizedWhenInUse {
            self.manager.startUpdatingLocation()
            self.currentLocation = self.manager.location
            self.getCurrentWeatherData()
        }

    }

    func getCurrentWeatherData() -> Void {

        var locationString: String = ""
        if let lat = self.currentLocation?.coordinate.latitude {
            locationString = "\(lat)"
        }
        if let lon = self.currentLocation?.coordinate.longitude {
            locationString += ", \(lon)"
        }


        let baseURL = NSURL(string: "https://api.forecast.io/forecast/\(apiKey)/")
        let forecastURL = NSURL(string: "\(locationString)", 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)
                // send to the dispatcher asycronously
                dispatch_async(dispatch_get_main_queue(), { () -> Void in
                    self.temperatureLabel.text = "\(currentWeather.temperature)"
                    self.iconView.image = currentWeather.icon!
                    self.currentTimeLabel.text = "At \(currentWeather.currentTime!) it is"
                    self.humidityLabel.text = "\(currentWeather.humidity)"
                    self.precipitationLabel.text = "\(currentWeather.precipProbability)"
                    self.summaryLabel.text = "\(currentWeather.summary)"

                    // Stop button animation
                    self.refreshActivityIndicator.stopAnimating()
                    self.refreshActivityIndicator.hidden = true
                    self.refreshButton.hidden = false

                })

            } else {
                let networkIssueController = UIAlertController(title: "Error", message: "Unable to load data due to connectivity issue", preferredStyle: .Alert)

                let okButton = UIAlertAction(title: "OK", style: .Default, handler: nil)
                networkIssueController.addAction(okButton)

                let cancelButton = UIAlertAction(title: "Cancel", style: .Cancel, handler: nil)
                networkIssueController.addAction(cancelButton)

                self.presentViewController(networkIssueController, animated: true, completion: nil)

                dispatch_async(dispatch_get_main_queue(), { () -> Void in
                    // Stop button animation
                    self.refreshActivityIndicator.stopAnimating()
                    self.refreshActivityIndicator.hidden = true
                    self.refreshButton.hidden = false
                })
            }

        })

        downloadTask.resume()

    }


    @IBAction func refresh() {

        getCurrentWeatherData()
        refreshButton.hidden = true
        refreshActivityIndicator.hidden = false
        refreshActivityIndicator.startAnimating()

    }


    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}