Nancy Cartwright

Nancy Cartwright is awesome ... slowly and slyly revealing to a kid on a mission he's lookin' at the voice of Bart Simpson. Nancy was at a shopping mall in Sherman Oaks, CA when she happened upon an 8th grader who was hawking sweets. You gotta see his reaction when the light bulb goes on.

Nancy Cartwright -- the voice of Bart Simpson -- is a defendant in a lawsuit involving her fiance's suicide, a failed construction project, and the Church of Scientology ... this according to documents obtained by TMZ. In the suit, the American Safety Casualty Insurance Company claims a…