[Reg 6.4.3->6.6] Application unusable after closing nested message boxes

#include <QApplication>
#include <QMessageBox>
#include <QPlainTextEdit>
#include <QPushButton>
#include <QVBoxLayout>

static void execMessage(const QString &text, QWidget *parent)
{
    QMessageBox mBox(parent);
    mBox.setText(text);
    mBox.setStandardButtons(QMessageBox::No | QMessageBox::Yes);
    mBox.setDefaultButton(QMessageBox::Yes);
    mBox.exec();
}

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QWidget w;
    w.setLayout(new QVBoxLayout);
    auto button = new QPushButton("Open Dialogs");
    w.layout()->addWidget(new QPlainTextEdit);
    w.layout()->addWidget(new QPlainTextEdit);
    w.layout()->addWidget(button);
    QObject::connect(button, &QPushButton::clicked, &w, [&] {
        QMetaObject::invokeMethod(
            &w,
            [] { execMessage("second message", QApplication::activeModalWidget()); },
            Qt::QueuedConnection);
        execMessage("first message", &w);
    });

    w.show();
    return a.exec();
}