title: How to detected dead conversation references when using proactive conversations
body: <p>We are using ContinueConversationAsync to send a Proactive Message to existing conversations which is working, what we want it a way to detect if that conversation is still active for instance if the conversation channel is webchat that session may no longer exists or a teams channel and the user has now left the organisation. Otherwise our ConversationReference table will just grow indefinably. At the moment SetProactiveMessage still just continues with no error even if there is no longer a user on the other end. </p>
  
  <pre><code> var conversationReference = botProactiveMessageConversation.ConversationReferenceJson.FromJson&lt;ConversationReference&gt;();
              conversationReference.ActivityId = null;
              MicrosoftAppCredentials.TrustServiceUrl(conversationReference.ServiceUrl);
              await defaultAdapter.ContinueConversationAsync(botProactiveMessageConversation.BotAppId, conversationReference, async (ITurnContext turnContext, CancellationToken cancellationToken) =&gt;
              {
                  turnContext.SetProactiveMessage(botProactiveMessageConversation.ProactiveMessageData);
                  await dialogBot.OnTurnAsync(turnContext, cancellationToken);
              }, default);
  </code></pre>