func uploadPhoto(completion: @escaping (_ url: String?) -> Void) {

    let photoName = NSUUID().uuidString
    let storageRef = Storage.storage().reference().child("photoURL").child("(photoName).jpg")

    let photo = self.photoImageView.image!

    if let uploadData = photo.jpegData(compressionQuality: 0.75) {
        storageRef.putData(uploadData, metadata: nil) { (metadata, error) in

            guard metadata != nil else {
                // Uh-oh, an error occurred!
                return
            }
            // Metadata contains file metadata such as size, content-type.
            //let size = metadata?.size

            // You can also access to download URL after upload.
            storageRef.downloadURL { (url, error) in
                guard url != nil else {
                    // Uh-oh, an error occurred!
                    return
                }

                // Metadata contains file metadata such as size, content-type, and download URL.

                // THROWS ERROR
                // let downloadURL = metadata.downloadURL

                // MY ATTEMPT
                var downloadURL = StorageReference.downloadURL(self.refStorage)
               completion(downloadURL = self.urlLabel.text)
            }
        }
    }
}