Update ConversationService.SendMessage.cs

Author: Oceania2018

src/Infrastructure/BotSharp.Core/Conversations/Services/ConversationService.SendMessage.cs

@@ -52,7 +52,10 @@ public async Task<bool> SendMessage(string agentId,
         message.Files?.Clear();
 
         // Save payload
-        message.Payload = string.IsNullOrEmpty(replyMessage.Payload) ? message.Content : replyMessage.Payload;
+        if (replyMessage != null)
+        {
+            message.Payload = string.IsNullOrEmpty(replyMessage.Payload) ? message.Content : replyMessage.Payload;
+        }
 
         // Before chat completion hook
         foreach (var hook in hooks)