Whenever I use a UserControl that contains a TabControl I get an “ApplicationException: Invalid Session”. This happens when I drag said UserControl from the Toolbox onto a Page, or when opening a page that already contains such a UserControl in the Designer. I think this bug was not always present because I already created multiple pages that contain such a UserControl, but I am now unable to open them. I’m not sure which version introduced this bug. I have attached a project with which I can reproduce this bug consistently.