title: Botframework DirectLineClient.Conversations.PostActivityAsync returns null
body: <p>I'm trying to send an activity through DirectLineClient library to my bot : </p>
  
  <pre><code>        var directLineClient = new DirectLineClient($"{secret}");
          directLineClient.BaseUri = new Uri($"https://directline.botframework.com/");
  
          var conversation = await directLineClient.Conversations.StartConversationAsync().ConfigureAwait(false);
          var activity = new Microsoft.Bot.Connector.DirectLine.Activity();
          activity.From = new Microsoft.Bot.Connector.DirectLine.ChannelAccount();
          activity.From.Name = "Morgan";
          activity.Text = message;
          activity.Type = "message";
  
          var resourceResponse = await directLineClient.Conversations.PostActivityAsync(conversation.ConversationId, activity).ConfigureAwait(false);
          await ReadBotMessagesAsync(directLineClient, conversation.ConversationId);
  </code></pre>
  
  <p>resourceResponse is always null.</p>
  
  <p><strong><em>Edit after Nicolas R answer</em></strong> </p>
  
  <p>I added a method to wait for a response from the bot : </p>
  
  <pre><code>    private static async Task ReadBotMessagesAsync(DirectLineClient client, string conversationId)
      {
          string watermark = null;
  
          while (true)
          {
              var activitySet = await client.Conversations.GetActivitiesAsync(conversationId, watermark);
              watermark = activitySet?.Watermark;
  
              foreach (Microsoft.Bot.Connector.DirectLine.Activity activity in activitySet.Activities)
              {
                  Console.WriteLine(activity.Text);
  
                  if (activity.Attachments != null)
                  {
                      foreach (Microsoft.Bot.Connector.DirectLine.Attachment attachment in activity.Attachments)
                      {
                          Console.WriteLine(attachment.ContentType);
                      }
                  }
              }
              if (activitySet.Activities.Count &gt; 0)
              {
                  return;
              }
              await Task.Delay(TimeSpan.FromSeconds(1)).ConfigureAwait(false);
          }
      }
  </code></pre>
  
  <p>But I never get out of ReadBotMessagesAsync.</p>
  
  <p>I precise that I can communicate with my bot through HTTP request (tested with Postman), and it should be sending a response message whenever a message is sent.</p>