[KPhotoAlbum] Cannot select tag for untagged images

[Christoph Moseler]

Hi all,

I cannot select a tag to be used for tagging newly imported images to 
kpa anymore.

Steps to reproduce:
Built kpa according 
https://community.kde.org/KPhotoAlbum/build_instructions under a newly 
installed kubuntu 20.04
Opened kpa and created a new DB
In settings->Tag Groups added a new category "Tags" and below a group 
"Untagged"
OK and saved DB
In settings->Categories below" Untagged Images" selected Category Tags 
created before.

Expected: Can select group "Untagged"
Actual: Tag-Combobox is empty and disabled.

The same is working in kpa 5.6.1.

Thanks,
Christoph