发送异步的Swift 3警告

I have this code:

我有这个代码：

DispatchQueue.global(priority: DispatchQueue.GlobalQueuePriority.default).async {
                let url = URL(string: itemImageURL )
                let data = try? Data(contentsOf: url!)
                if data != nil {
                    DispatchQueue.main.async{
                        cell.advImage!.image = UIImage(data: data!)
                    }
                }
            }
DispatchQueue.glo