Side bar background has weird effect on Big Sur

I am trying to adopt the new MacOS Big Sur sidebar style in one of my apps.
Previously I chose a source list and added the items to it which worked fine.
I have selected the new template Window Controller with Sidebar in Xcode.
When I run the project without adding any items or controls, it looks fine. However, if I add the Source List, the background of the side bar has some weird effect. It seems like the Column of the Source List messes up the background.

How can I add items to a sidebar on MacOS Big Sur? I am running Xcode 12 beta 3 and the latest beta 3 of MacOS Big Sur - 11.0 Beta (20A5323l).

Unfortunately I can not post a screenshot.
Side bar background has weird effect on Big Sur
 
 
Q