Cant add widget to viewport

Im trying to add a simple widget to the viewport inside its c++ class but it doesnt work.
I made a C++ class (UCPP_Inventory) based on UserWidget and then made a Blueprint (WBP_Inventory) that is based on my c++ class. Now when I’m trying to create the widget and add it to the viewport under NativeConstruct() it just doesnt add it. Trying to solve this for quite some time now, and nothing works that I found on the internet.

UCPP_Inventory.h

UCPP_Inventory.cpp

What am I missing? WBP_Inventory looks like this (an image and a uniform grid panel):