我使用国内线路上传文件完全正常，把配置切换到美国线路之后就无法上传文件，一直报错，错误信息如下：
NullReferenceException: Object reference not set to an instance of an object
LeanCloud.AVFile+<>c__DisplayClass34_0.b__0 (System.Threading.Tasks.Task toAwait)
LeanCloud.Storage.Internal.TaskQueue.Enqueue[Task1] (System.Func2 taskStart, CancellationToken cancellationToken)
LeanCloud.AVFile.SaveAsync (IProgress`1 progress, CancellationToken cancellationToken)
LeanCloud.AVFile.SaveAsync ()