I noticed the following error messages in the IDE console window. (Puzzle1To8:23445): GLib-CRITICAL **: g_ascii_strup: assertion `str != NULL' failed (Puzzle1To8:23445): Gdk-CRITICAL **: IA__gdk_keyval_from_name: assertion `keyval_name != NULL' failed I was clicking on the squares and typing in a number when these messages appeared.