PLT-5284 Fix webhook notifications for channel creator is not in (#5119)

* Fix webhook notifications for channel creator is not in

* Fix unit test

app/post.go

@@ -99,10 +99,6 @@ func handlePostEvents(post *model.Post, teamId string, triggerWebhooks bool) *mo
 		channel = result.Data.(*model.Channel)
 	}
 
-	if _, err := SendNotifications(post, team, channel); err != nil {
-		return err
-	}
-
 	var user *model.User
 	if result := <-uchan; result.Err != nil {
 		return result.Err
@@ -110,6 +106,10 @@ func handlePostEvents(post *model.Post, teamId string, triggerWebhooks bool) *mo
 		user = result.Data.(*model.User)
 	}
 
+	if _, err := SendNotifications(post, team, channel, user); err != nil {
+		return err
+	}
+
 	if triggerWebhooks {
 		go func() {
 			if err := handleWebhookEvents(post, team, channel, user); err != nil {