Fix focusObjectDuringDestruction crash

Don’t assume that there is an active modal widget
on QApplication::focusChanged(). That signal can also
be emitted on focus clear during dialog destruction.

Task-number: QTBUG-66536
Change-Id: I20c64339c56a52b39c26a7683b62779deba576d9
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

tests/auto/widgets/dialogs/qfiledialog/tst_qfiledialog.cpp

@@ -1521,7 +1521,8 @@ public:
         const QWindow *window = QGuiApplication::topLevelWindows().constFirst();
 
         const QFileDialog *fileDialog = qobject_cast<QFileDialog*>(QApplication::activeModalWidget());
-        QVERIFY(fileDialog);
+        if (!fileDialog)
+            return;
 
         // The problem in QTBUG-57193 was from a platform input context plugin that was
         // connected to QWindow::focusObjectChanged(), and consequently accessed the focus