When you create an empty level and just add the UMG_Inventory to the viewport (custom GameMode, override map settings, set PawnClass to none, CreateWidget inside level blueprint), does it still show up as invalid?
What happens if you add a delay before the IsValid node?
Where/when do you check if it is valid (in your “game flow”)?
Thank you so much for your reply. Now, it finally works. An Engine restart fixed the problem. From the code-side of things, it all seems right. Have a nice weekend!