厚度动画麻烦?(带有Begin函数参数)

有一个主框架，上面有一个按钮和控件，还有stackpanel。试图将工作动画从xaml移动到。cs，我写了下一个很酷的函数)):

private void ToggleButton_Checked(object sender, RoutedEventArgs e)
        {
int heightBottom = 100;
System.Windows.Thickness th = stackPanel1.Margin;
            th.Top = stackPanel1.Margin.Top + heightBottom;
ThicknessAnimation animStackPanel1 = new ThicknessAnimation
            {
                From = stackPanel1.Margin,
                To = th,
                AccelerationRatio = 0.2,
                FillBehavior = FillBehavior.Stop,
                DecelerationRatio = 0.8,
                Duration = DURATION
            };
System.Windows.Thickness th2 = fxPSEditorView.Margin;
            th2.Right = fxPSEditorView.Margin.Right - heightBottom;
            th2.Bottom = fxPSEditorView.Margin.Bottom - heightBottom;
            ThicknessAnimation animPSEditorView = new ThicknessAnimation
            {
                From = fxPSEditorView.Margin,
                To = th2,
                AccelerationRatio = 0.2,
                FillBehavior = FillBehavior.Stop,
                DecelerationRatio = 0.8,
                Duration = DURATION
            };
Storyboard.SetTarget(animPSEditorView, fxPSEditorView);
            Storyboard.SetTargetProperty(fxPSEditorView, new PropertyPath(MarginProperty));
            sb.Children.Add(animPSEditorView);
Storyboard.SetTarget(animStackPanel1, stackPanel1);
            Storyboard.SetTargetProperty(stackPanel1, new PropertyPath(MarginProperty));
            sb.Children.Add(animStackPanel1);
sb.Begin();//null reference error here! 
};

据我所知，我必须为sb.Begin指定两个参数-一个用于stackPanel1，另一个用于fxPSEditorView。但是它不把一组对象作为第一个参数。任何想法如何运行这个动画将是受欢迎的!谢谢你