Custom Editor, button stays disabled

        <DataTemplate x:Key="selectClausePropertyGridTemplate">
            <Grid>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="*" />
                </Grid.ColumnDefinitions>
                <shared:PopupButton Grid.Column="0" Content="..." DisplayMode="ButtonOnly" 
                            HorizontalAlignment="Right" IsTransparencyModeEnabled="True" 
                            Command="{x:Static dbc:DataBrowseControl.ShowGroupByDialog}" 
                            CommandParameter="{Binding RelativeSource={RelativeSource 

AncestorType={x:Type propgrid:IPropertyDataAccessor}}}" />
            </Grid>
        </DataTemplate>

        [Category("Aggregate settings"),
         DisplayName("Group by field data"),
         Description("Additional group by clause."),
         Editor(typeof(SelectClauseUIEditor), typeof(PropertyEditor)),
        DefaultValue("")]
        public string GroupByFieldDataClause
        {
            get { return _groupByFieldDataClause; }
            set
            { 
                _groupByFieldDataClause = value;
                if (PropertyChanged != null)
                    PropertyChanged(this, new PropertyChangedEventArgs("GroupByFieldDataClause"));
            }
        }
 

    public class SelectClauseUIEditor : PropertyEditor
    {
        public SelectClauseUIEditor()
        {
        }

        public override DataTemplate ValueTemplate
        {
            get
            {
                DataTemplate dt = (DataTemplate)Application.Current.Resources["selectClausePropertyGridTemplate"];
                return dt;
            }
        }
    }

        static DataBrowseControl()
        {
            // Initialize routed commands
            DataBrowseControl.ShowGroupByDialog = new RoutedCommand("ShowGroupByDialog", typeof                (DataBrowseControl));
            CommandManager.RegisterClassCommandBinding(typeof(DataBrowseControl),
                new CommandBinding(DataBrowseControl.ShowGroupByDialog,         OnShowGroupByDialogExecute, OnShowGroupByDialogCanExecute));
        }

      private static void OnShowGroupByDialogExecute(object sender, ExecutedRoutedEventArgs e)
        {
            IPropertyDataAccessor property = e.Parameter as IPropertyDataAccessor;
            if (null != property)
            {
                MessageBox.Show("Do whatever you want to do"); 
                /*
                MyColorDialog dlg = new MyColorDialog();
                if (property.Value is Color)
                    dlg.Color = (Color)property.Value;
                if (true == dlg.ShowDialog())
                    property.Value = dlg.Color;
                 */
                e.Handled = true;
            }
        }

        /// <summary>
        /// Occurs when the <see cref="RoutedCommand"/> needs to determine whether it can execute.
        /// </summary>
        /// <param name="sender">The sender of the event.</param>
        /// <param name="e">A <see cref="CanExecuteRoutedEventArgs"/> that contains the event data.</param>
        private static void OnShowGroupByDialogCanExecute(object sender, CanExecuteRoutedEventArgs e)
        {
            if (e.Parameter is IPropertyDataAccessor)
            {
                e.CanExecute = true;
                e.Handled = true;
            }
        }