gradients widget example crashes

uint ShadeWidget::colorAt(int x)
{
    generateShade();

    QPolygonF pts = m_hoverPoints->points();
    for (int i = 1; i < pts.size(); ++i) {
        if (pts.at(i - 1).x() <= x && pts.at(i).x() >= x) {
            QLineF l(pts.at(i - 1), pts.at(i));
            l.setLength(l.length() * ((x - l.x1()) / l.dx())); // dx is 0, I think?
            return m_shade.pixel(qRound(qMin(l.x2(), (qreal(m_shade.width() - 1)))),
                                 qRound(qMin(l.y2(), qreal(m_shade.height() - 1))));
        }
    }
    return 0;
}

	Locals		
		i	1	int
		l	@0x7fffffffc9b0	QLineF
			pt1	(0.0, 0.0)	QPointF
			pt2	(0.0, -0.0)	QPointF
		pts	<2 items>	QPolygonF
			[0]	(0.0, 0.0)	QPointF
			[1]	(0.0, -0.0)	QPointF
		this	@0x5555558ec990	ShadeWidget
			[QWidget]	@0x5555558ec990	QWidget
			[d]	@0x555555a727f0	QWidgetPrivate
			[parent]	@0x55555590fa40	GradientEditor
			[children]	<1 items>	QList<QObject*>
			[properties]	<at least 0 items>	
			[methods]	<1 items>	
			[extra]		
			m_alpha_gradient	@0x5555558ec9e0	QLinearGradient
			m_hoverPoints	@0x55555578a800	HoverPoints
				[QObject]	@0x55555578a800	QObject
				[d]	@0x555555945720	QObjectPrivate
				[parent]	@0x5555558ec990	ShadeWidget
				[children]	<0 items>	QList<QObject*>
				[properties]	<at least 0 items>	
				[methods]	<3 items>	
				[extra]		
				m_bounds	0.0x0.0+0.0+0.0	QRectF
				m_connectionPen	@0x55555578a8a8	QPen
				m_connectionType	HoverPoints::CurveConnection (2)	HoverPoints::ConnectionType
				m_currentIndex	-1	int
				m_editable	true	bool
				m_enabled	true	bool
				m_fingerPointMapping	<0 items>	QHash<int, int>
				m_locks	<2 items>	QList<unsigned int>
				m_pointBrush	@0x55555578a8a0	QBrush
				m_pointPen	@0x55555578a898	QPen
				m_pointSize	(11.0, 11.0)	QSizeF
				m_points	<2 items>	QPolygonF
					[0]	(0.0, 0.0)	QPointF
					[1]	(0.0, -0.0)	QPointF
				m_shape	HoverPoints::CircleShape (0)	HoverPoints::PointShape
				m_sortType	HoverPoints::XSort (1)	HoverPoints::SortType
				m_widget	@0x5555558ec990	ShadeWidget
				staticMetaObject	@0x5555555a8fe0	QMetaObject
			m_shade	(140x40)	QImage
			m_shade_type	ShadeWidget::RedShade (0)	ShadeWidget::ShadeType
			staticMetaObject	@0x5555555a9160	QMetaObject
		x	0	int
	Inspector		
	Expressions		
	Return Value		
	Tooltip		
		m_shade	(140x40)	QImage

1  raise                                                                           
2  abort                                                                           
3  qt_message_fatal                                      qlogging.cpp         1892 
4  QMessageLogger::fatal                                 qlogging.cpp         890  
5  qt_assert                                             qglobal.cpp          3114 
6  QLineF::setLength                                     qline.h              375  
7  ShadeWidget::colorAt                                  gradients.cpp        108  
8  GradientEditor::pointsUpdated                         gradients.cpp        229  
9  QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPr qobjectdefs_impl.h   152  
10 QtPrivate::FunctionPointer<void (GradientEditor:: *)( qobjectdefs_impl.h   185  
11 QtPrivate::QSlotObject<void (GradientEditor:: *)(), Q qobjectdefs_impl.h   395  
12 QtPrivate::QSlotObjectBase::call                      qobjectdefs_impl.h   375  
13 doActivate<false>                                     qobject.cpp          3822 
14 QMetaObject::activate                                 qobject.cpp          3882 
15 ShadeWidget::colorsChanged                            moc_gradients.cpp    135  
16 QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPr qobjectdefs_impl.h   152  
17 QtPrivate::FunctionPointer<void (ShadeWidget:: *)()>: qobjectdefs_impl.h   185  
18 QtPrivate::QSlotObject<void (ShadeWidget:: *)(), QtPr qobjectdefs_impl.h   395  
19 QtPrivate::QSlotObjectBase::call                      qobjectdefs_impl.h   375  
20 doActivate<false>                                     qobject.cpp          3822 
21 QMetaObject::activate                                 qobject.cpp          3882 
22 HoverPoints::pointsChanged                            moc_hoverpoints.cpp  151  
23 HoverPoints::firePointChange                          hoverpoints.cpp      423  
24 HoverPoints::eventFilter                              hoverpoints.cpp      262  
25 QCoreApplicationPrivate::sendThroughObjectEventFilter qcoreapplication.cpp 1165 
26 QApplicationPrivate::notify_helper                    qapplication.cpp     3389 
27 QApplication::notify                                  qapplication.cpp     3345 
28 QCoreApplication::notifyInternal2                     qcoreapplication.cpp 1039 
29 QCoreApplication::sendEvent                           qcoreapplication.cpp 1447