ThemeResource on TreeTable showing String instead of Icon(image)


I have a TreeTable and I want to show an Icon for some items in a column, but instead of the icon, a String with the path is shown on the table!

Does anybody knows why?

This is where I define the column:

hc.addContainerProperty("icon", Resource.class, null);

This is where I set the icon:

permissions.getItem(spu).getItemProperty("icon").setValue(new ThemeResource("../runo/icons/16/ok.png"));