Workaround for crash on derefWindow on application shutdown

Ideally there shouldn't be nullptrs in the list. This fix will however
prevent crashes, until we find out why.

Fixes: QTBUG-116426
Pick-to: 6.6.2 6.5
Change-Id: I2c85ad73832ab4d8b9992b4d8e661758fae85ac7
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 5d79b207c7)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit fe0b983fc5)

src/quick/items/qquickitem.cpp

@@ -3045,8 +3045,8 @@ void QQuickItemPrivate::derefWindow()
     paintNode = nullptr;
 
     for (int ii = 0; ii < childItems.size(); ++ii) {
-        QQuickItem *child = childItems.at(ii);
-        QQuickItemPrivate::get(child)->derefWindow();
+        if (QQuickItem *child = childItems.at(ii))
+            QQuickItemPrivate::get(child)->derefWindow();
     }
 
     dirty(Window);