[TreeView] Midlevel node collapse results in invalid index request

When you open multiple levels and then try to collapse a node that is more than one level up the last opened node (but not at the top level). You hit this function:

int QQuickTreeModelAdaptor::lastChildIndex(const QModelIndex &index)
{
    if (!m_expandedItems.contains(index))
        return itemIndex(index);

    QModelIndex parent = index.parent();
    QModelIndex nextSiblingIndex;
    while (parent.isValid()) {
        nextSiblingIndex = parent.sibling(parent.row() + 1, 0);
        if (nextSiblingIndex.isValid())
            break;
        parent = parent.parent();
    }

    int firstIndex = nextSiblingIndex.isValid() ? itemIndex(nextSiblingIndex) : m_items.count();
    return firstIndex - 1;
}

Specifically this line: `nextSiblingIndex = parent.sibling(parent.row() + 1, 0);` has the possibility to request an invalid index (one that is out of range). Since the model adaptor can just call `rowCount` I don't see immediately what is the reason to request an invalid index here.

I would like to throw an exception if an invalid index is requested because I assume Qt code will be careful not to do that so I want to catch any bugs in my own code where I might be requesting an invalid index.