Wizards: Do not crash when dialogs are still open on quit

Change-Id: I49622484e316b902699e5b63ea3eea827d2f2680
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/plugins/coreplugin/windowsupport.h

@@ -85,6 +85,7 @@ private:
     QAction *m_zoomAction;
     QAction *m_closeAction;
     QAction *m_toggleFullScreenAction;
+    bool m_shutdown = false;
 };
 
 } // Internal