在两个viewController之间传递数据失败

Question

我正在尝试将categoriesTableViewController中的数据传递给quotesContentViewController.Here是代码：

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    let DestViewController : quotesContentsViewController = segue.destinationViewController as! quotesContentsViewController

    let categoryString = sender?.titleLabel!!.text
    let quotesContentURLString = "http://quotes.rest/qod.json?category=" + categoryString!

    if let quotesContentURL = NSURL(string: quotesContentURLString) {
        self.updateQuotesContentFeed(quotesContentURL, completion: {(feed) -> Void in
            //print(feed!.item.imageURL)*
            DestViewController.feed = feed!
            DestViewController.author.text = feed!.item.author
            DestViewController.quotes.text = feed!.item.quotes
            DestViewController.imageURLString = feed!.item.imageURL

            //print ("updateQuotesContentFeed")*
            //print(DestViewController.imageURLString)*
        })
    }

}

DestViewController.author.text和DestViewController.quotes.text已成功传递到quotesContentViewController，但DestViewController.imageURLString失败。 print(DestViewController.imageURLString)打印出我想要的内容。（这就是为什么我认为它应该已成功通过）

现在代码quotesContentViewController：

import UIKit

class quotesContentsViewController: UIViewController {

    var feed: quotesContentFeed?

    var urlSession: NSURLSession!

    weak var dataTask: NSURLSessionDataTask?

    var imageURLString: String!

    @IBOutlet var quotes: UILabel!
    @IBOutlet var author: UILabel!

    @IBOutlet var backgroundImage: UIImageView!

    override func viewDidLoad() {

        //let request = NSURLRequest(URL: self.imageURL!)

        //dataTask = self.urlSession.dataTaskWithRequest(request) {(data, response, error) -> Void //in
            //NSOperationQueue.mainQueue().addOperationWithBlock({ () -> Void in
                //if error == nil && data != nil {
                    //let image = UIImage(data: data!)
                    //self.backgroundImage.image = image
                //}
            //})
        //}
        print("viewDidLoad")
        print(self.imageURLString)


        super.viewDidLoad()

        // Do any additional setup after loading the view.
    }

当我运行它时，它会抛出错误fatal error: unexpectedly found nil while unwrapping an Optional value。您可以查看下面错误的屏幕截图。