[regression] setting windows flags in constructor leading to crash

When WIndows flags are set to be frameless in QDialog, this some time crashes.

Attached example can be used to crash it on windows as well on Mac, 

Steps, run the code and hit Menu->test 

(it may crash here on Mac)

this will show frame-less dialog, click any where in dialog to close it. (it may crash here )

This crash is not reproducible with every try but it is reproducible with most .

This works good with Qt5.10.1 but broken in 5.11.0. 

Stack 

Qt5Cored.dll!QObject::isWidgetType() Line 142 
qobject_cast<QWidget *>(QObject * o) Line 742 
updateObjects(const QList<QObject const *> & objects) Line 2732 
QStyleSheetStyle::repolish(QWidget * w) Line 2911 
QWidgetPrivate::inheritStyle() Line 2823 
QWidget::setParent(QWidget * parent, QFlags<enum Qt::WindowType> f) Line 10748 
Qt5Widgetsd.dll!QWidgetPrivate::setWindowFlags(QFlags<enum Qt::WindowType> flags) Line 10558 
Qt5Widgetsd.dll!QWidget::setWindowFlags(QFlags<enum Qt::WindowType> flags) Line 10520 
crash_test.exe!DlgAboutNew::DlgAboutNew(QWidget * parent) Line 14 
crash_test.exe!MainWindow::on_actiontest_triggered() Line 20