I have discovered that WidgetKit 3.1.0 conflicts with Regular Labs tabs and slider editor buttons. I have raised this with Yootheme, but posting here in case anyone else experiences the same problem and is looking for a solution.
When updating or installing WidgetKit 3.1.0 the WidgetKit admin interface is completely blank when Regular Labs tabs and sliders editor buttons are published. Unpublish the buttons and the WidgetKit admin interface returns.
Previous versions of WidgetKit did not have this issue.