QHBoxLayout crash with non-contiguous indexes of layout items

CustomWidget::CustomWidget(QWidget *parent)
    : QWidget{parent}
{
    QHBoxLayout * l = new QHBoxLayout(this);
    l->setSpacing(0);

    // BUG: Without the following line, this code crashes in Qt6.
    // It does not crash in Qt5 and behaves as if there is a zero-size
    // empty space (much like the following line).

//    l->insertSpacing(0,0);

    l->insertWidget(1, new QLabel("foo", this));
    l->insertWidget(2, new QLabel("bar", this));

    // this will crash
    QSize s = l->sizeHint();
}