Fix GTK warnings when destroying unattached wxMenuBar.

Reset m_focusWidget to NULL when destroying m_widget in wxMenuBar dtor,
otherwise we try to use this already destroyed (because it's the same as
m_widget) widget in wxWindow dtor later resulting in critical GTK warnings.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72559 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/menu.cpp

@@ -68,6 +68,7 @@ wxMenuBar::~wxMenuBar()
         // Work around a probable bug in Ubuntu 12.04 which causes a warning if
         // gtk_widget_destroy() is called on a wxMenuBar attached to a frame
         GtkWidget* widget = m_widget;
+        m_focusWidget =
         m_widget = NULL;
         g_object_unref(widget);
     }