experiment suddenly ends

[mpg9]

All I want is to have a black screen alternate with a white screen until the participant presses the spacebar. I have 3 lines, one with the black screen, one with the white, and the third line is a blank screen. I have skipping functions so that the two trials repeat until the spacebar is pressed. When I run it, I see trial 1, trial 2, then a flash of something, then the experiment closes. What's going on?