Crashing and getting an error off of the code from video

@IBOutlet weak var currentTemperatureLabel: UILabel?
@IBOutlet weak var currentWeatherIcon: UIImageView?
@IBOutlet weak var currentPrecipitationLabel: UILabel?
@IBOutlet weak var currentTemperatureRangeLabel: UILabel?

// Location coordinates
let coordinate: (lat: Double, lon: Double) = (37.8267,-122.423)

// TODO: Enter your API key here
private let forecastAPIKey = ""

var weeklyWeather: [DailyWeather] = []

override func viewDidLoad() {
    super.viewDidLoad()
    configureView()

    retrieveWeatherForecast()
}

func configureView() {
    // Set table view's background view property
    tableView.backgroundView = BackgroundView()

    // Change the font and size of nav bar text
    if let navBarFont = UIFont(name: "HelveticaNeue-Thin", size: 20.0) {
        let navBarAttributesDictionary: [NSObject: AnyObject]? = [
            NSForegroundColorAttributeName: UIColor.whiteColor(),
            NSFontAttributeName: navBarFont
        ]
        navigationController?.navigationBar.titleTextAttributes = navBarAttributesDictionary
    }
}

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

// MARK: - Table view data source

override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
    // Return the number of sections.
    return 1
}

override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    // Return the number of rows in the section.
    return weeklyWeather.count
}

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCellWithIdentifier("WeatherCell") as! UITableViewCell

    let dailyWeather = weeklyWeather[indexPath.row]
    cell.textLabel?.text = dailyWeather.day
    return cell
}

// MARK: - Weather Fetching

func retrieveWeatherForecast() {
    let forecastService = ForecastService(APIKey: forecastAPIKey)
    forecastService.getForecast(coordinate.lat, lon: coordinate.lon) {
        (let forecast) in

        if let weatherForecast = forecast,
            let currentWeather = weatherForecast.currentWeather {

                dispatch_async(dispatch_get_main_queue()) {

                    if let temperature = currentWeather.temperature {
                        self.currentTemperatureLabel?.text = "\(temperature)º"
                    }

                    if let precipitation = currentWeather.precipProbability {
                        self.currentPrecipitationLabel?.text = "Rain: \(precipitation)%"
                    }

                    if let icon = currentWeather.icon {
                        self.currentWeatherIcon?.image = icon
                    }

                    self.weeklyWeather = weatherForecast.weekly

                    if let highTemp = self.weeklyWeather.first?.maxTemperature,
                        let lowTemp = self.weeklyWeather.first?.minTemperature {
                            self.currentTemperatureRangeLabel?.text = "↑\(highTemp)º↓\(lowTemp)º"
                    }

                    self.tableView.reloadData()
                }
        }
    }
}