RadMessageBox - SetThemeName method is not working correctly, after dispose on owner form.

Workaround - Reset the radMessageBoxForm field via reflection. For example:

            var field = typeof(RadMessageBox).GetField("radMessageBoxForm", BindingFlags.NonPublic | BindingFlags.Static);
            field.SetValue(null, null);

            RadMessageBox.SetThemeName("Office2007Black");