isNotification is required

lib/form/comment.js

@@ -10,6 +10,6 @@ module.exports = form(
   field('commentForm.comment_position').trim().toInt(),
   field('commentForm.is_markdown').trim().toBooleanStrict(),
 
-  field('slackNotificationForm.isNotification').trim().toBooleanStrict(),
+  field('slackNotificationForm.isNotification').trim().toBooleanStrict().required(),
   field('slackNotificationForm.notifSlackChannel').trim(),
 );