Failed to see how can this snippet of code produced an unhandled exception on a some watches

Question

Hi, I got 3 occurences of Unhandled exception at the "if (mDragStartY < height / 5)" line (line 10). mDragStartY is a class variable. Is it possible to get a 'stop' but no 'start' event? Any other reason?

    function onDrag(dragEvent ) {
        var coord = dragEvent.getCoordinates();

        if (dragEvent.getType() == WatchUi.DRAG_TYPE_START) {
            mDragStartX = coord[0];
            mDragStartY = coord[1];
        }
        else if (dragEvent.getType() == WatchUi.DRAG_TYPE_STOP) {
            var height = System.getDeviceSettings().screenHeight;
            if (mDragStartY < height / 5) { // 'Swiped' the answer portion, only test if left or right
                if (gDataEntry) {
                    return true; // No digit resizing while entering data
                }
                
                gDigitsChanged = true;

                if (mDragStartX > coord[0]) {  // 'Swiped' left
                    gDigits++;
                    if (gDigits > 8) {
                        gDigits = 8;
                    }
                }
                else {   // 'Swiped' right