Fix rendering announcement everywhere

[MAILPOET-1591]

assets/js/src/settings/announcement.jsx

@@ -4,10 +4,11 @@ import Announcement from './new_subscriber_announcement.jsx';
 
 const container = document.getElementById('new_subscriber_announcement');
 
-
+if (container) {
   ReactDOM.render(
     <Announcement
       installedAt={window.mailpoet_installed_at}
       imageUrl={window.mailpoet_new_subscriber_announcement_image}
     />, container
   );
+}