Crash when accessing non-existent parentWidget

The function QWidget::setVisible(bool) in qwidget.cpp is not implemented uniformly in regard to the parentWidget() property, sometimes there is a pointer check on the return value of this function and sometimes there isn't. Especially line 8250 caused a crash in our case:

 if (isWindow() || parentWidget()->isVisible()) {

should be:

 if (isWindow() || (parentWidget() && parentWidget()->isVisible())) {

There are more occurences of using the return value of parentWidget() without a pointer check in QWidget::serVisible(bool):

QWidget *pw = parentWidget();
if (!testAttribute(Qt::WA_WState_Created)
    && (isWindow() || pw->testAttribute(Qt::WA_WState_Created))) {
        create();
}

and

// adjust size if necessary
        if (!wasResized
            && (isWindow() || !parentWidget()>d_func()>layout))  {