At least in CppunitTest_smoektest there is a case where
SystemWindow::setOptimalLayoutSize tries to downcast a VclGrid as a VclBox
(stack top below), and that even appears to be unnecessary as the only place
that requires the downcast is the Dialog::settingOptimalLayoutSize override.  So
maybe it is guaranteed that GetWindow(WINDOW_FIRSTCHILD) is a VclBox if this is
a Dialog, but need not be the case if this is not a Dialog?  Or the real bug is
elsewhere...

SystemWindow::setOptimalLayoutSize()
SystemWindow::DoInitialLayout()
FloatingWindow::StateChanged(StateChangedType)
SfxFloatingWindow::StateChanged(StateChangedType)
SmToolBoxWindow::StateChanged(StateChangedType)
vcl::Window::ImplCallInitShow()
vcl::Window::Show(bool, unsigned short)
SfxChildWindow::Show(unsigned short)
SfxWorkWindow::HidePopups_Impl(bool, bool, unsigned short)
SfxDispatcher::DoActivate_Impl(bool, SfxViewFrame*)
SfxViewFrame::DoActivate(bool, SfxViewFrame*)
SfxApplication::SetViewFrame_Impl(SfxViewFrame*)
SfxViewFrame::SetViewFrame(SfxViewFrame*)
SfxViewFrame::MakeActive_Impl(bool)
SfxBaseController::ConnectSfxFrame_Impl(SfxBaseController::ConnectSfxFrame)
SfxBaseController::attachFrame(com::sun::star::uno::Reference<com::sun::star::frame::XFrame> const&)
(anonymous namespace)::SfxFrameLoader_Impl::impl_createDocumentView(com::sun::star::uno::Reference<com::sun::star::frame::XModel2> const&, com::sun::star::uno::Reference<com::sun::star::frame::XFrame> const&, comphelper::NamedValueCollection const&, rtl::OUString const&)
(anonymous namespace)::SfxFrameLoader_Impl::load(com::sun::star::uno::Sequence<com::sun::star::beans::PropertyValue> const&, com::sun::star::uno::Reference<com::sun::star::frame::XFrame> const&)
framework::LoadEnv::impl_loadContent()
framework::LoadEnv::startLoading()
framework::LoadEnv::loadComponentFromURL(com::sun::star::uno::Reference<com::sun::star::frame::XComponentLoader> const&, com::sun::star::uno::Reference<com::sun::star::uno::XComponentContext> const&, rtl::OUString const&, rtl::OUString const&, int, com::sun::star::uno::Sequence<com::sun::star::beans::PropertyValue> const&)
...

Change-Id: I786ad451fdf73eca22860695a614ee5b889a83cd