问调取接口报错，求助！？

    <Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="GradientBrushAnimation.MainWindow"
    x:Name="Window"
    Title="MainWindow"
    Width="640" Height="480">
    <Window.Resources>
        <Storyboard x:Key="Storyboard1">
            <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[0].(GradientStop.Color)" Storyboard.TargetName="rectangle">
                <EasingColorKeyFrame KeyTime="0:0:2" Value="Red"/>
            </ColorAnimationUsingKeyFrames>
            <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[1].(GradientStop.Color)" Storyboard.TargetName="rectangle">
                <EasingColorKeyFrame KeyTime="0:0:2" Value="#FF71FF00"/>
            </ColorAnimationUsingKeyFrames>
        </Storyboard>
    </Window.Resources>
    <Window.Triggers>
        <EventTrigger RoutedEvent="FrameworkElement.Loaded">
            <BeginStoryboard Storyboard="{StaticResource Storyboard1}"/>
        </EventTrigger>
    </Window.Triggers>

    <Grid x:Name="LayoutRoot">
        <Rectangle x:Name="rectangle" Margin="78,102,292,144" Stroke="Black">
            <Rectangle.Fill>
                <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                    <GradientStop Color="Black" Offset="0"/>
                    <GradientStop Color="White" Offset="1"/>
                </LinearGradientBrush>
            </Rectangle.Fill>
        </Rectangle>
    </Grid>
</Window>

public class BrushAnimation : AnimationTimeline
{
    public override Type TargetPropertyType
    {
        get
        {
            return typeof(Brush);
        }
    }

    public override object GetCurrentValue(object defaultOriginValue,
                                           object defaultDestinationValue,
                                           AnimationClock animationClock)
    {
        return GetCurrentValue(defaultOriginValue as Brush,
                               defaultDestinationValue as Brush,
                               animationClock);
    }
    public object GetCurrentValue(Brush defaultOriginValue,
                                  Brush defaultDestinationValue,
                                  AnimationClock animationClock)
    {
        if (!animationClock.CurrentProgress.HasValue)
            return Brushes.Transparent;

        //use the standard values if From and To are not set 
        //(it is the value of the given property)
        defaultOriginValue = this.From ?? defaultOriginValue;
        defaultDestinationValue = this.To ?? defaultDestinationValue;

        if (animationClock.CurrentProgress.Value == 0)
            return defaultOriginValue;
        if (animationClock.CurrentProgress.Value == 1)
            return defaultDestinationValue;

        return new VisualBrush(new Border()
        {
            Width = 1,
            Height = 1,
            Background = defaultOriginValue,
            Child = new Border()
            {
                Background = defaultDestinationValue,
                Opacity = animationClock.CurrentProgress.Value,
            }
        });
    }

    protected override Freezable CreateInstanceCore()
    {
        return new BrushAnimation();
    }

    //we must define From and To, AnimationTimeline does not have this properties
    public Brush From
    {
        get { return (Brush)GetValue(FromProperty); }
        set { SetValue(FromProperty, value); }
    }
    public Brush To
    {
        get { return (Brush)GetValue(ToProperty); }
        set { SetValue(ToProperty, value); }
    }

    public static readonly DependencyProperty FromProperty =
        DependencyProperty.Register("From", typeof(Brush), typeof(BrushAnimation));
    public static readonly DependencyProperty ToProperty =
        DependencyProperty.Register("To", typeof(Brush), typeof(BrushAnimation));
}

<EventTrigger RoutedEvent="Loaded">
    <BeginStoryboard>
        <Storyboard >
            <local:BrushAnimation Storyboard.TargetName="border"
                                  Storyboard.TargetProperty="Background" 
                                  Duration="0:0:5" From="Red" 
                                  RepeatBehavior="Forever" AutoReverse="True" >
                <local:BrushAnimation.To>
                    <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                        <GradientStop Color="#FF00FF2E" Offset="0.005"/>
                        <GradientStop Color="#FFC5FF00" Offset="1"/>
                        <GradientStop Color="Blue" Offset="0.43"/>
                    </LinearGradientBrush>
                </local:BrushAnimation.To>
            </local:BrushAnimation>
        </Storyboard>
    </BeginStoryboard>
</EventTrigger>

var animation = new BrushAnimation
{
    From = Brushes.Red,
    To = new LinearGradientBrush (Colors.Green, Colors.Yellow, 45),
    Duration = new Duration(TimeSpan.FromSeconds(5)),
};
animation.Completed += new EventHandler(animation_Completed);
Storyboard.SetTarget(animation, border);
Storyboard.SetTargetProperty(animation, new PropertyPath("Background"));

var sb = new Storyboard();
sb.Children.Add(animation);
sb.Begin();