Drawing contours detected in image quickly turns into an all-black screen

Hi, I've been reading tutorials and I'm trying to detect rectangles from my webcam using OpenCV. When I run this code, I see white specs on parts of the screen for a few frames. These white areas flicker in a familiar pattern to what I've seen image recognition do. Within 2 seconds the screen is entirely black. Why is this happening? I'm trying to detect rectangles on the screen. Thanks!