对象引用未设置为 Teams 频道中对象的实例

问：

它在 Bot 框架模拟器中完美运行，但在这一点上在 Teams 中显示错误

if(Dialog != null)
{
  await Dialog.RunAsync( turnContext, ConversationState.CreateProperty<DialogState>("DialogState"), cancellationToken);
  //Logger.LogInformation("Running dialog with Message Activity.");
}  

代码如下：

protected override async Task OnMessageActivityAsync(
  ITurnContext<IMessageActivity> turnContext, 
  CancellationToken cancellationToken)
{
  // Run the Dialog with the new message Activity.
  if (Dialog == null)
    throw new ArgumentNullException("Dialog is null");
  if (ConversationState == null)
    throw new ArgumentNullException("ConversationState is null");
  if (cancellationToken == null)
    throw new ArgumentNullException("cancellationToken is null");

  if (turnContext == null)
    throw new ArgumentNullException("turnContext is null");
  if(Dialog != null)
  {
    //error
    await Dialog.RunAsync( 
      turnContext, 
      ConversationState.CreateProperty<DialogState>("DialogState"), 
      cancellationToken);
    //Logger.LogInformation("Running dialog with Message Activity.");
  }         
}