FCM Net threw an error

[Chinajiyong]

Following is my code:
public async Task SendPushNotification(string deviceToken, Notification notification, object data)
{
var serverKey = _configuration["FCMServerKey"];

        bool sent = false;

        if (!String.IsNullOrEmpty(deviceToken))
        {
            using (var sender = new Sender(serverKey))
            {
                var message = new Message
                {
                    To = deviceToken,
                    Priority = Priority.High,
                    ContentAvailable = true,
                    Notification = notification,
                    Data = data
                };

                var result = await sender.SendAsync(message);
                Console.WriteLine($"Success: {result.MessageResponse.Success}");

                sent = result.MessageResponse.Success == 1;
            }
        }

        return sent;
    }

And I got this error:

at Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware.Invoke(HttpContext context)
at Microsoft.AspNetCore.Server.IISIntegration.IISMiddleware.Invoke(HttpContext httpContext)
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequests[TContext](IHttpApplication1 application) 2018-12-10 15:12:17.6160|Default|0HLIUDJT5GOSP:00000007||Microsoft.AspNetCore.Server.Kestrel|ERROR|Connection id "0HLIUDJT5GOSP", Request id "0HLIUDJT5GOSP:00000007": An unhandled exception was thrown by the application. System.OperationCanceledException: The operation was canceled. at System.Net.Http.HttpClient.HandleFinishSendAsyncError(Exception e, CancellationTokenSource cts) at System.Net.Http.HttpClient.FinishSendAsyncBuffered(Task1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)
at FCM.Net.Sender.SendAsync(HttpContent content)
at FCM.Net.Sender.SendAsync(Message message)
at Nova.Api.Tools.FCMPushNotification.SendPushNotification(String deviceToken, Notification notification, Object data) in E:\NovaGitProjects\Nurish\Nurish\SourceCode\Server\src\Nova.Pms.Modules\Nova.Api\Tools\FCM\FCMPushNotification.cs:line 81
at Nova.Api.Controllers.PlanController.RespondActivity(RespondRequestDto request) in E:\NovaGitProjects\Nurish\Nurish\SourceCode\Server\src\Nova.Pms.Modules\Nova.Api\Controllers\PlanController.cs:line 281

[Chinajiyong]

@angelobelchior Could you please take a look at this error?