AKs of category with unset color are invisible in scheduling view

I set up a test instance and added AK categories but first left the color fields for the categories empty. If I then visit the scheduling view, i.e. localhost:8000/admin/AKModel/event/<event_name>/schedule/ there are no AKs visible on the right hand side (under the tab "Unscheduled"), however hovering over them shows corresponding tooltips.

If I then set a color string for the category, the categories are shown with white text on that background color.

Without further research, I suspect that the default background color is chosen as white, leading to the view rendering white text on white background.

I used the current commit of the main branch in a debugging setup.