While creating a 2D sketch (in part workspace): Trying to trim an arc. AD wouldn't do it. Yes, they are all on the same plane, etc, etc. Then after a few attempts I get the 'reference not set to an instance of an object' error message. In sketch mode. After that every time I move the mouse the screen flickers. A lot. As in fit-inducing strobe type flickering. Even when mousing over the background the screen flickers.