Widgets: check if parent exists before calling parent->locale()

If a top level, parentless widget has Qt::WA_WindowPropagation
set, Qt will crash when trying to resolve the widgets locale.
The reason is that we try to access the QLocale of the
non-existing parent.

This patch will add a check if a parent exists before trying
to access it.

Task-number: QTBUG-61213
Change-Id: I09a6351a12dc1fffab3069b70e3d7b3932317c85
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/widgets/kernel/qwidget.cpp

@@ -6007,9 +6007,9 @@ void QWidgetPrivate::resolveLocale()
     Q_Q(const QWidget);
 
     if (!q->testAttribute(Qt::WA_SetLocale)) {
-        setLocale_helper(q->isWindow() && !q->testAttribute(Qt::WA_WindowPropagation)
-                            ? QLocale()
-                            : q->parentWidget()->locale());
+        QWidget *parent = q->parentWidget();
+        setLocale_helper(!parent || (q->isWindow() && !q->testAttribute(Qt::WA_WindowPropagation))
+                         ? QLocale() : parent->locale());
     }
 }