Crash when set QGraphicsOpacityEffect opacity to 1.0 if child widgets have graphics effect

Use the simple code below to create a new application. The application crashes when starting. The reason is that the child widgets have graphics effect, and the paint device will fail to be created when the second graphics effect acquiring the context. So a null painterdevice is used and the application crashes.

Note that this happens only when set opacity of QGraphicsOpacityEffect to 1.0. When effect is full opacity, QGraphicsOpacityEffect uses a different code path than translucent.

#include <QtGui>
int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    QWidget* w1 = new QWidget();
    QWidget* w2 = new QWidget( w1 );

    QGraphicsOpacityEffect* e1 = new  QGraphicsOpacityEffect();
    QGraphicsDropShadowEffect* e2 = new  QGraphicsDropShadowEffect();
    e1->setOpacity( 1.0 ); // 0.9 works
    w1->setGraphicsEffect( e1 );
    w2->setGraphicsEffect( e2 );
    w1->show();
    
    return app.exec();
}