Crash in QQuickWidget on a simple qml project when setResizeMode/size is not set

//c++ part
#include <QApplication>
#include <QQuickWidget>


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QQuickWidget* w= new QQuickWidget();
    w->setSource(QUrl::fromLocalFile("w.qml"));
    //w->setResizeMode(QQuickWidget::SizeRootObjectToView);
    //w->resize(400,400);
    w->show();


    return a.exec();
}


//qml part
import QtQuick 2.0

Rectangle {
    id:root
}