qt: fix crash on exit with CompositorWin7

The window fed to `InterfaceWindowHandler`
needs to outlive the handler, because the
handler accesses the window during its
destruction.

modules/gui/qt/maininterface/compositor_win7.cpp

@@ -123,6 +123,7 @@ void CompositorWin7::destroyMainInterface()
 
 void CompositorWin7::unloadGUI()
 {
+    m_interfaceWindowHandler.reset();
     m_qmlView.reset();
     commonGUIDestroy();
 }