I'm using Blender for a couple of years now, and I recently installed Blender 2.8. I want to add a circle of 6 vertices, but when I add a circle, the 'Add Circle' dialog is disabled (screenshot below), and I can't change anything. This happens with every object I add to the scene. I haven't experienced anything like this in previous editions of Blender, and I can't find any setting to enable it. I've tried to restart Blender, and even reinstall it, but that didn't help. Is there any setting I'm missing that might be causing this?