Fix the case where threadtimestamp is empty

Author: ccrims0n

response.go

@@ -34,7 +34,11 @@ func (r *response) ReportError(err error, options ...ReportErrorOption) {
 	event := r.botCtx.Event()
 	message := rtm.NewOutgoingMessage(fmt.Sprintf(errorFormat, err.Error()), event.Channel)
 	if defaults.ThreadResponse {
-		message.ThreadTimestamp = event.ThreadTimestamp
+		if event.ThreadTimestamp != "" {
+			message.ThreadTimestamp = event.ThreadTimestamp
+		} else {
+			message.ThreadTimestamp = event.EventTimestamp
+		}
 	}
 
 	rtm.SendMessage(message)
@@ -54,14 +58,19 @@ func (r *response) Reply(message string, options ...ReplyOption) error {
 	rtm := r.botCtx.RTM()
 	event := r.botCtx.Event()
 	if defaults.ThreadResponse {
+		if event.ThreadTimestamp != "" {
+			threadTimestamp = event.ThreadTimestamp
+		} else {
+			threadTimestamp = event.EventTimestamp
+		}
 		_, _, err := rtm.PostMessage(
 			event.Channel,
 			slack.MsgOptionText(message, false),
 			slack.MsgOptionUser(rtm.GetInfo().User.ID),
 			slack.MsgOptionAsUser(true),
 			slack.MsgOptionAttachments(defaults.Attachments...),
 			slack.MsgOptionBlocks(defaults.Blocks...),
-			slack.MsgOptionTS(event.ThreadTimestamp),
+			slack.MsgOptionTS(threadTimestamp),
 		)
 		return err
 	}