gtk_gesture_single_get_current_sequence can return NULL, but sequence functions won't accept it
gtk_gesture_single_get_current_sequence can return NULL in certain circumstances. In C, one can pass this NULL sequence value to functions like gtk_gesture_get_last_event and the right thing will happen, but this usage is impossible with pygobject, since gtk_gesture_get_last_event will reject a None as an invalid argument.