无法从Firebase的URL下载图像

Question

我无法从用户输入中下载图片。每当我尝试上传图像时，都会出现错误屏幕。我正在使用Swift 4.0的Xcode 9。

if let imgData = UIImageJPEGRepresentation(img, 0.2){
    let imgUid = NSUUID().uuidString
    let metadata = StorageMetadata()
    metadata.contentType = "image/jpeg"
    let storageItem = Storage.storage().reference().child(imgUid)
    storageItem.putData(imgData, metadata: metadata) { (metadata, error) in
        if error != nil {
            print("Couldn't Upload Image")
        } else {
            print("Uploaded")
            storageItem.downloadURL(completion: { (url, error) in
                if error != nil {
                    print(error!)
                    return
                }
                if url != nil {
                    self.setUser(img: url!.absoluteString)
                }
            }
        )
        }
    }
}