QTreeWidgetItem::setText() will not update the widget

#include <QtGui>

class MainWindow : public QMainWindow {
    Q_OBJECT

public slots:
    void changeModelValue(int);

public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow() {};

private:
    QTreeWidget *treeWidget;
    QTreeWidgetItem *item;
};

MainWindow::MainWindow(QWidget *parent) :  QMainWindow(parent)
{
    treeWidget = new QTreeWidget;
    treeWidget->setColumnCount(2);

    QSlider *slider = new QSlider;

    QHBoxLayout *hlayout = new QHBoxLayout;
    hlayout->addWidget(treeWidget);
    hlayout->addWidget(slider);
    QWidget *mwidget = new QWidget;
    setCentralWidget(mwidget);
    centralWidget()->setLayout(hlayout);

    QStringList strList;
    strList << "variable1" << "0";
    item = new QTreeWidgetItem(strList);

    treeWidget->insertTopLevelItem(0, item);

    connect(slider, SIGNAL(valueChanged(int)), this, SLOT(changeModelValue(int)));
}

void MainWindow::changeModelValue(int i)
{
    item->setText(1, QString::number(i));
}

#include "main.moc"

int main(int argc, char **argv)
{
    QApplication app(argc, argv);
    MainWindow window;
    window.show();
    return app.exec();
}