So the workflow for the modular menu system is like this: First you create the menu, as shown above, which gives you an instance of that (empty) menu. It is not shown on the screen yet. Then you add elements to that menu instance, as you want, like buttons etc. Later you can show (and hide) that menu instance to the screen – you can do so anytime you want, either directly after creating the menu, or at any later time.
By UMG default you cannot add the same menu instance to the viewport multiple times, thus you cannot (accidently) show the same menu instance on the screen twice.
I hope that helps a bit to understand the menu system works. Please feel free to ask further questions if you have more of them. I am happy to share as much information as you want!
Thanks man 