iOS swift Apple在TestFlight Review期间报告我的应用程序崩溃

Question

连续第二次在Apple的TestFlight评论中遭到拒绝。这次从他们那里得到了崩溃日志，显然我的应用程序看起来正在崩溃异步图像下载和缓存（在UITableView中使用）。几次检查代码，但仍然不知道什么是错的，应用程序在没有崩溃的设备和模拟器上工作，有和没有互联网连接。请看一下突出显示的崩溃线：

var imgCache = [String: UIImage]()

func downloadAndCacheImg(url: String?, completion: ((UIImage)->())?) {
    if url == nil { return }
    if let img = imgCache[url!] {
        completion?(img)
        return
    }
    if let img = UIImage(contentsOfFile: formatFileNameForCachedImg(url!)) {
        self.imgCache[url!] = img
        completion?(img)
        return
    }
    if let nsurl = NSURL(string: url!) {
        NSURLConnection.sendAsynchronousRequest(NSURLRequest(URL: nsurl), queue: NSOperationQueue.mainQueue(), completionHandler: { (response, data, error) -> Void in
            if let img = UIImage(data: data) {
                self.imgCache[url!] = img    <<<<< CRASH HERE
                data.writeToFile(self.formatFileNameForCachedImg(url!), atomically: true)
                completion?(img)
            }
        })
    }
}

func formatFileNameForCachedImg(url: String) -> String {
    let dir = NSSearchPathForDirectoriesInDomains(.CachesDirectory, .UserDomainMask, true)
    let dirName = dir[0] as! String
    var fname = url.stringByReplacingOccurrencesOfString("/", withString: "", options: nil, range: nil)
    let out = String(format: "%@/%@", dirName, fname)
    return out
}