SignalR核心控制台客户端未收到通知

我环顾了一些类似的问题，却没有弄清楚这个问题。我的.NET核心Web API项目中有一个简单的Hub。这是集线器：

 public class NotificationHub : Hub<INotificationClient>
{
  public async Task SendMessage(string user, string msg)
    {
        await Clients.All.ReceiveMessage(user, msg);
    }
    public Task SendMessageToCaller(string msg)
    {
        return Clients.Caller.ReceiveMessage(msg);
    }

    public Task SendMessageToPartner(string user, string msg)
    {
        return Clients.Client(user).ReceiveMessageToPartner(msg);
    }
}

这是接口：

 public interface INotificationClient
{
    Task ReceiveMessage(string user, string msg);
    Task ReceiveMessage(string msg);
    Task ReceiveMessageToPartner( string msg);

}

这是来自控制器的代码：

[Route("[controller]")]
[ApiController]
public class NotificationsController : ControllerBase
{
    private IHubContext<NotificationHub> _hub;

    public NotificationsController(IHubContext<NotificationHub> hub)
    {
        _hub = hub;

    }
    [HttpGet]
    public async Task<IActionResult> Get()
    {
        var msg = new NotificationData { ClientId = "12345", Notification = "Somone just connected" };
        await _hub.Clients.All.SendAsync("Notification", msg);
        return Ok(new { Message = "Request complete" });
    }
}

最后是控制台客户端代码：

 Console.WriteLine("Press a key to start listening");
        Console.ReadKey();

        Console.WriteLine("Client Listening!");
        var connection = new HubConnectionBuilder()
           .WithUrl("http://localhost:61514/notifications")

           .Build();


        connection.On<NotificationData>("Notification", (notificationData) =>
            Console.WriteLine($"Somebody connected: {notificationData.ClientId}"));

        connection.StartAsync().GetAwaiter().GetResult();

        Console.WriteLine("Listening. Press a key to quit");
        Console.ReadKey();

这是带有映射的Web应用程序的启动：

 public void ConfigureServices(IServiceCollection services)
    {
        services.AddSignalR();
        services.AddControllers();
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseRouting();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
            endpoints.MapHub<NotificationHub>("/Notifications");
        });
    }

我不断收到此错误：System.IO.IOException：'在开始握手之前，服务器已断开连接。在这里，我一定会丢失一些东西。

更新：
打开日志并收到此错误：

dbug：Microsoft.AspNetCore.Http.Connections.Client.HttpConnection [8]与位于http：// localhost：61514 / notifications的服务器建立连接。dbug：Microsoft.AspNetCore.Http.Connections.Client.HttpConnection [9]与服务器建立连接'BdROAEEQnGUeDYAW5EspRA'。dbug：Microsoft.AspNetCore.Http.Connections.Client.HttpConnection [7]使用网址开始传输“ ServerSentEvents”：http：// localhost：61514 / notifications。信息：Microsoft.AspNetCore.Http.Connections.Client.Internal.ServerSentEventsTransport [1]开始传输。传输方式：文本。dbug：Microsoft.AspNetCore.Http.Connections.Client.Internal.ServerSentEventsTransport [3]开始接收循环。dbug：Microsoft.AspNetCore.Http.Connections.Client.Internal.ServerSentEventsTransport [9]收到30个字节。解析SSE框架。dbug：Microsoft.AspNetCore.Http.Connections.Client.Internal.ServerSentEventsTransport [4]接收循环已停止。dbug：Microsoft.AspNetCore.Http.Connections.Client.Internal.ServerSentEventsTransport [100]启动发送循环。dbug：Microsoft.AspNetCore.Http.Connections.Client.HttpConnection [18]传输“ ServerSentEvents”已启动。dbug：Microsoft.AspNetCore.Http.Connections.Client.Internal.ServerSentEventsTransport [102]发送循环已取消。dbug：Microsoft.AspNetCore.Http。Connections.Client.Internal.ServerSentEventsTransport [101]发送循环已停止。未处理的异常。信息：Microsoft.AspNetCore.Http.Connections.Client.HttpConnection [3] HttpConnection已启动。信息：Microsoft.AspNetCore.SignalR.Client.HubConnection [24]使用HubProtocol'json v1'。System.IO.IOException：在握手开始之前，服务器已断开连接。在Microsoft.AspNetCore.SignalR.Client.HubConnection.StartAsyncCore（CancellationToken cancellingToken） ），位于System.Threading的Microsoft.AspNetCore.SignalR.Client.HubConnection.StartAsyncInner（CancellationToken cancelleToken）处。