nice, figured it out.
technically I was trying to focus on the widget and not the button that’s inside the widget.
for me it was obvious what I wanted to do, set the focus on the button, but I should have told the engine
that as well 
so, here’s the solution.
out from the created widget I got the button, set that to a variable, so I could get it from the inventory widget
when setting the focus inside the player bp. and it works 