QWebEngineView with Qt::Window flag not closed when its parent is closed

In the construction function of main window:

QWebEngineView *w =newQWebEngineView(this);
w->setWindowFlags(w->windowFlags() | Qt::Window);
w->load(QUrl("https://qt.io");

Later, after I close the main window(the only primary window), the window of w is still open. It should be closed and the app should exit.