serializing only aadObjectId's

bots/messaging_extension_action_preview_bot.py

@@ -461,7 +461,7 @@ class TeamsMessagingExtensionsActionPreviewBot(TeamsActivityHandler):
                        conversation_id=conversation_id)
             )
             members = await self.adapter.get_conversation_members(turn_context)
-            members = [Account.get_schema().dump(x) for x in members]
+            members = [Account.load(x).aad_object_id for x in members]
             await turn_context.send_activity(str(members))
             Log.d(TAG, "on_message_activity::help_resp: {}".format(response))
             return