Cannot modify TinyMCEConfiguration by implement ExtendedXhtmlStringEditorDescriptor

Hi,

I want to extend XhtmlStringEditorDescriptor follow below code with purpose to get theme value from startpage and it throw error. How can I solve this?

[EditorDescriptorRegistration(TargetType = typeof(XhtmlString))]
    public class ExtendedXhtmlStringEditorDescriptor : XhtmlStringEditorDescriptor
    {
        private readonly ServiceAccessor<TinyMceConfiguration> _tinyMceConfiguration;
        private readonly IContentRepository _contentRepository;
        public ExtendedXhtmlStringEditorDescriptor(ServiceAccessor<TinyMceConfiguration> tinyMceConfiguration, IContentRepository contentRepository) : base(tinyMceConfiguration)
        {
            _tinyMceConfiguration = tinyMceConfiguration;
            _contentRepository = contentRepository;
        }
        public override void ModifyMetadata(ExtendedMetadata metadata, IEnumerable<Attribute> attributes)
        {
            base.ModifyMetadata(metadata, attributes);
            var settings = metadata.EditorConfiguration["settings"] as TinyMceSettings;
            if (settings != null)
            {
                var startPage = _contentRepository.Get<StartPage>(SiteDefinition.Current.StartPage);
                settings.BodyClass(startPage.Theme.ToString());
            }
            metadata.EditorConfiguration["settings"] = settings;
        }
    }