[MM-32253] Fix "Cannot mark unread a DM message in a thread" (#16794)

Automatic Merge

app/post.go

@@ -1343,10 +1343,6 @@ func (a *App) MaxPostSize() int {
 
 // countThreadMentions returns the number of times the user is mentioned in a specified thread after the timestamp.
 func (a *App) countThreadMentions(user *model.User, post *model.Post, teamId string, timestamp int64) (int64, *model.AppError) {
-	team, err := a.GetTeam(teamId)
-	if err != nil {
-		return 0, err
-	}
 	channel, err := a.GetChannel(post.ChannelId)
 	if err != nil {
 		return 0, err
@@ -1379,6 +1375,14 @@ func (a *App) countThreadMentions(user *model.User, post *model.Post, teamId str
 		return int64(count), nil
 	}
 
+	var team *model.Team
+	if teamId != "" {
+		team, err = a.GetTeam(teamId)
+		if err != nil {
+			return 0, err
+		}
+	}
+
 	groups, nErr := a.getGroupsAllowedForReferenceInChannel(channel, team)
 	if nErr != nil {
 		return 0, model.NewAppError("countMentionsFromPost", "app.channel.count_posts_since.app_error", nil, nErr.Error(), http.StatusInternalServerError)