Button images not showing

[RenePhilipse]

I am using the treeview widget in one of my 5.21.3 projects. Basically it works fine, but I have just one issue. When I create a button (in the widget settings 4-Actions) in the header of the treeview and add a image to it (in tab 2-Button), it is not showing in the frontend of the application. The button is rendered and the caption is showing but simply not the image.

For reference I also tried the treeview demo project in 5.21.3 and there the button image is also not showing. Same goes for version 5.15.1