Qt 显示树的根元素

我想创建自己的树模型，结果是这样的：

如何使根元素显示出来？

class TreeItem
{
public:
    explicit TreeItem(const QString &data, TreeItem* parent = 0);
    ~TreeItem();

    void appendChild(TreeItem *child);
    void setParent(TreeItem *parent);


    TreeItem *child(int row);
    int childCount() const;
    QString data() const;
    int row() const;
    TreeItem *parentItem();

private:
    QList<TreeItem*> _childItems;
    QString _data;
    TreeItem* _parentItem;
};

class TreeModel : public QAbstractItemModel
{
public:
    explicit TreeModel( const QString &data, QObject *parent = 0 );
    ~TreeModel();

    QVariant data( const QModelIndex &index, int role ) const override;
    QVariant headerData(int section, Qt::Orientation orientation, int role) const;
    Qt::ItemFlags flags( const QModelIndex &index ) const override;
    QModelIndex index( int row, int column, const QModelIndex &parent = QModelIndex() ) const override;
    QModelIndex parent( const QModelIndex &index)  const override;
    int rowCount( const QModelIndex &parent = QModelIndex() ) const    override;
    int columnCount( const QModelIndex &parent = QModelIndex() ) const override;

private:

    TreeItem* _rootItem;
};


QModelIndex TreeModel::index(int row, int column, const QModelIndex &parent ) const
{
    qDebug() << "TreeModel::index";
    if ( !hasIndex(row, column, parent) )
        return QModelIndex();

    TreeItem* parentItem;

    if ( !parent.isValid() )
        parentItem = _rootItem;
    else
        parentItem = static_cast<TreeItem*>( parent.internalPointer() );

    TreeItem* childItem = parentItem->child(row);
    if (childItem)
        return createIndex(row, column, childItem);
    else
        return QModelIndex();
}

QModelIndex TreeModel::parent(const QModelIndex &index) const
{
    qDebug() << "TreeModel::parent";
    if (!index.isValid())
        return QModelIndex();

    TreeItem *childItem = static_cast<TreeItem*>(index.internalPointer());
    TreeItem *parentItem = childItem->parentItem();

    if (parentItem == _rootItem)
        return QModelIndex();

    return createIndex(parentItem->row(), 0, parentItem);
}


MainWindow::MainWindow()
{
    mainWindow.setupUi(this);

    TreeModel* model = new TreeModel("abc");

    mainWindow.treeView->setModel(model);
}