问 默认值不能是“Unset”

EN

我有一个附加属性，当我将它附加到xaml元素时，我得到一个名为 的异常“默认值不能'Unset'"

我的附加属性逻辑

    public static DataTemplate GetFooterContentTemplate(DependencyObject obj)
        return (DataTemplate)obj.GetValue(FooterContentTemplateProperty);
    public static void SetFooterContentTemplate(DependencyObject obj, DataTemplate value)
        obj.SetValue(FooterContentTemplateProperty, value);
    // Using a DependencyProperty as the backing store for GetFooterContentTemplate.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty FooterContentTemplateProperty =
        DependencyProperty.Register("FooterContentTemplate", typeof(DataTemplate), typeof(RadWindowFooterProperties), new PropertyMetadata(DependencyProperty.UnsetValue));
    public static Object GetFooterContent(DependencyObject obj)
        return (Object)obj.GetValue(FooterContentProperty);
    public static void SetFooterContent(DependencyObject obj, Object value)
        obj.SetValue(FooterContentProperty, value);
    // Using a DependencyProperty as the backing store for RadWindowFooterContent.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty FooterContentProperty =
        DependencyProperty.RegisterAttached("FooterContent", typeof(Object), typeof(RadWindowFooterProperties), new PropertyMetadata(DependencyProperty.UnsetValue));

我的XAML：

     <Grid x:Name="FooterRoot"  Grid.Row="2" MinHeight="42">
                        <Border Background="{StaticResource ShellFooterBrush}"/>
                        <Border Background="{DynamicResource ShellTileBrush}"/>
                        <ContentPresenter Content="{Binding Path=(Local:RadWindowFooterProperties.FooterContent),RelativeSource={RelativeSource Mode=TemplatedParent}}" ContentTemplate="{Binding Path=(Local:RadWindowFooterProperties.FooterContentTemplate),RelativeSource={RelativeSource Mode=TemplatedParent}}" RecognizesAccessKey="True"/>
                    </Grid>

请让我知道我在哪里做错了。