QModelIndex is invalid in QTreeView based on Qt SimpleTreeModel example (source code pasted)

[qt_fan_4k]

I'm creating a file browser based on the Qt4.8 example SimpleTree
And I rewrote my own setUpModelData as shown in the function FileTreeModel::setupModelData below (totally less than 60 lines)

Basically, the argument root contains the user-specified root path, like "D:\qt_project\prj1", and setUpModelData will create a tree model from the real root directory D:\ to this user-specified path, with all sibling items added.

The problem is, whenever I expanded any directory under D:, I got a segment fault.
With all the addresses dumped, I could see that the childItem in the function TreeModel::parent is none of the dumped address, i.e., such childItem retrieved from index.internalPointer is invalid.

How come internalPointer returns an invalid TreeItem pointer ?

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

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

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

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

QModelIndex FileTreeModel::setupModelData(TreeItem *root) {
    QFileInfo fi_root = QFileInfo(root->data().toString());
    TreeItem *activeItem = root;    // always points to the current child
    QDir it_dir = fi_root.dir();    // check with the locating dir
    QList<TreeItem *> lstParents;     // to calculate the model index
    bool rootDirNotMet = true;
    do {
        TreeItem *startItem = new TreeItem(it_dir.dirName());      // create the father node
        startItem->setData(QVariant(it_dir.path()));
        startItem->setIcon(QIcon(":/images/parent"));
        lstParents.append(activeItem);
        QString addr; addr.setNum((long)(void *)activeItem, 16);
        qDebug() << (activeItem->data().toString() + " @@ " + addr);    // dump all address for each path
        this->listSubDir(it_dir, startItem, activeItem);
        activeItem = startItem;
        if (!rootDirNotMet) break;   // ensure the root dir is also processed when cdUp failed
        rootDirNotMet = it_dir.cdUp();
    }  while(1);
    qDebug() << it_dir.absolutePath();
    activeItem->setParent(rootItem);             // setParent sets parentItem in the TreeItem
    activeItem->setText(it_dir.absolutePath());
    rootItem->appendChild(activeItem);
    QModelIndex itemIndex = QModelIndex(); // index(0, 0, QModelIndex());        // calculate from root index
    do {
        itemIndex = this->index(lstParents.takeLast()->row(), 0, itemIndex);
    } while (!lstParents.isEmpty());
    return itemIndex;   // return the index of the user-specified root
}   // setupModelData

void FileTreeModel::listSubDir(QDir &dir, TreeItem *parent, TreeItem *registered_child = 0) {
    if (parent->childCount() > 1) return;       // this dir has already been listed
    if (parent->hasChildren() && parent->child(0)->text().compare("")) return;  // the only child is not dummy
    QDirIterator it(dir.path(), QDirIterator::NoIteratorFlags);      // check this parent node
    while (it.hasNext()) {      // check each child
        QString full_path;
        QFileInfo fi = QFileInfo(full_path = it.next());
        if (!fi.fileName().compare(".") || !fi.fileName().compare("..")) continue;        // ignore "." and ".."
        TreeItem *siblingItem;
        if (registered_child && registered_child->data().toString() == full_path) {     // this is registered child
            siblingItem = registered_child;
            parent->appendChild(siblingItem);
        } else if (parent->childCount() == 1 && parent->child(0)->text().compare("") == 0) { // can reuse the dummy item
            siblingItem = parent->child(0);
            siblingItem->setText(fi.fileName());
        } else {    // need a new node for the item
            siblingItem = new TreeItem(fi.fileName());
            parent->appendChild(siblingItem);
        }   // else for a new node
        if (fi.isDir()) {  // is directory
            siblingItem->setIcon(QIcon(":/images/parent"));
            TreeItem *dummyItem = new TreeItem("");     // dummy child to mark the folder to expand
            siblingItem->appendChild(dummyItem);
        } else {    // a file
            siblingItem->setIcon(QIcon(":/images/child"));
        } // else of if dir
        siblingItem->setData(full_path);
        siblingItem->setParent(parent);
        QString addr; addr.setNum((long)(void *)siblingItem, 16);
        qDebug() << (siblingItem->data().toString() + " ## " + addr);    // dump all address for each found child
    }   // while it.hasNext
}   // listSubDir

[qt_fan_4k]

The reason that I added dummy child is to make all directories expandable, a lazy expansion.

[qt_fan_4k]

The dummy child is the culprit. It did not set the parent so the parent is invalid.