Two connected QSortFilterProxyModel(s) can crash an application

#include <QtGui>

class Widget : public QWidget {
    Q_OBJECT

public slots:
    void swap();

public:
    Widget(QWidget *parent = 0);

private:
    QTreeView *treeView;
    QStandardItemModel *model1;
    QStandardItemModel *model2;
    QSortFilterProxyModel *proxyModel;
    QSortFilterProxyModel *toggleProxy;
};

Widget::Widget(QWidget *parent) : QWidget(parent)
{
    model1 = new QStandardItemModel;
    QStandardItem *parentItem = model1->invisibleRootItem();
    for (int i = 0; i < 4; ++i) {
        QStandardItem *item = new QStandardItem(QString("model1 item %0").arg(i));
        parentItem->appendRow(item);
        parentItem = item;
    }

    model2 = new QStandardItemModel;
    QStandardItem *parentItem2 = model2->invisibleRootItem();
    for (int i = 0; i < 4; ++i) {
        QStandardItem *item = new QStandardItem(QString("model2 item %0").arg(i));
        parentItem2->appendRow(item);
        parentItem2 = item;
    }

    toggleProxy = new QSortFilterProxyModel;
    toggleProxy->setSourceModel(model1);

    proxyModel = new QSortFilterProxyModel;
    proxyModel->setSourceModel(toggleProxy);

    QHBoxLayout *layout = new QHBoxLayout;
    treeView = new QTreeView;
//    treeView->setModel(toggleProxy);
    treeView->setModel(proxyModel);
    layout->addWidget(treeView);
    QPushButton *swapButton = new QPushButton("Swap");
    layout->addWidget(swapButton);
    setLayout(layout);

    connect(swapButton, SIGNAL(clicked()), this, SLOT(swap()));
}

void Widget::swap()
{
    if (toggleProxy->sourceModel() == model1)
        toggleProxy->setSourceModel(model2);
    else
        toggleProxy->setSourceModel(model1);
}

#include "main.moc"

int main(int argc, char **argv)
{
    QApplication a(argc, argv);
    Widget w;
    w.show();
    return a.exec();
}