Default FSlateBrush Resource

i want to set a default FSlateBrush resource which in this case is “/Engine/Content/Editor/Slate/Common/SpinArrows.png”

when i open up the editor everything looks fine:

however as soon as i compile the userwidget, the FSlateBrush switches to this:

when i press the “revert” button right next to the image, the arrows come back again but i have to compile to play which switches it back to the white again.

does anyone have an idea why this happens?
thanks in advance!