// 1. create and show a QWaylandXdgSurfaceV6::Popup (e.g. QMenu),
parented to parent.
auto pMenu = new QMenu("test", parent);
pMenu->show();

// 2. close the parent.
QTimer::singleShot(100, [parent]() {parent->close(); });

// 3. Next time a popup is shown it will crash in
QWaylandXdgSurfaceV6::setGrabPopup.

It seems a popup has a m_parent pointer to it's parent surface. Assuming the popup is closed first - the code works.
But if the parent surface is closed first - the surface does not seem to have any
knowledge of it's child popup(s) - and does nothing to destroy them first.

In previous QT versions and windowing systems could be expected that closing a parent window will automatically close any child windows (especially without crashing).

It seems to happen also only when the window decorations are disabled.