Inventory

So i am watching inventory tutorials and created simple inventory.
But now i want my own inventory system.
It is actually simple.

So i need inventory open only from UI. I open UI click button opens new window and here is inventory. How would you approach to this?

Tutorial: https://www.youtube.com/watch?v=yxqSkFNAzE0&list=PL4G2bSPE_8uktjEdP4ZuRq5r2o4JMdZfM

Click button -> Create Widget -> Add to Viewport

Yeah did that.
So ill try in detail.
In tutorial you can open inventory in level trough player character. What i want is to get same inventory but in different level where character does not exist. Its almost like from main menu. So before game itself begins and character is spawned i want it to be able to see inventory before game has started. I did get it to work but now i need to transferre items from one level to another. What i collected in one map it goes to different map.

Either keep the inventory in the Game Instance or / and look into Save Game Objects - you will need to load / save the game anyway eventually, right?