Gui: avoid that toolbars of active workbench become invisible

Forum thread: https://forum.freecadweb.org/viewtopic.php?f=8&t=67748

src/Gui/MainWindow.cpp

@@ -1176,7 +1176,14 @@ void MainWindow::closeEvent (QCloseEvent * e)
 
         /*emit*/ mainWindowClosed();
         d->activityTimer->stop();
-        saveWindowSettings();
+
+        // https://forum.freecadweb.org/viewtopic.php?f=8&t=67748
+        // When the session manager jumps in it can happen that the closeEvent()
+        // function is triggered twice and for the second call the main window might be
+        // invisible. In this case the window settings shouldn't be saved.
+        if (isVisible())
+            saveWindowSettings();
+
         delete d->assistant;
         d->assistant = nullptr;