C# WPF移动网格错误

我在尝试新的东西。我有几个网格，当我试图抓取一个网格时，我想把它的父网格移到顶部网格周围。此时，我在

m_startOffset=newvector（translate.X，translate.Y）上得到一个System.NullReferenceException。有人知道我应该如何解决这个问题吗

用户界面：

在XAML中声明父网格的RenderTransform：

<Grid x:Name="GRLogin" Margin="1401,292,0,0" HorizontalAlignment="Left" VerticalAlignment="Top" Width="501" d:IsHidden="True" Focusable="True">
    <Grid.RenderTransform>
        <TranslateTransform/>
    </Grid.RenderTransform>
    ...
</Grid>


...

您还可以检查RenderTransform是否为null，在这种情况下，您可以将其设置为：
element.RenderTransform=new TranslateTransform（）
谢谢，出于某种奇怪的原因，当我这样做时，它什么也做不了。看起来te程序没有响应，因为它不会对任何其他输入做出反应。
    private void Grid_MouseDown(object sender, MouseButtonEventArgs e)
    {
        FrameworkElement element = ((Grid)sender).Parent as Grid;
        TranslateTransform translate = element.RenderTransform as TranslateTransform;

        m_start = e.GetPosition(GridHost);
        m_startOffset = new Vector(translate.X, translate.Y);
        element.CaptureMouse();
    }

    private void Grid_MouseMove(object sender, MouseEventArgs e)
    {
        FrameworkElement element = ((Grid)sender).Parent as Grid;
        TranslateTransform translate = element.RenderTransform as TranslateTransform;

        if (element.IsMouseCaptured)
        {
            Vector offset = Point.Subtract(e.GetPosition(GridHost), m_start);

            translate.X = m_startOffset.X + offset.X;
            translate.Y = m_startOffset.Y + offset.Y;
        }
    }

    private void Grid_MouseUp(object sender, MouseButtonEventArgs e)
    {
        FrameworkElement element = ((Grid)sender).Parent as Grid;
        element.ReleaseMouseCapture();
    }

<Grid x:Name="GRLogin" Margin="1401,292,0,0" HorizontalAlignment="Left" VerticalAlignment="Top" Width="501" d:IsHidden="True" Focusable="True">
    <Grid.RenderTransform>
        <TranslateTransform/>
    </Grid.RenderTransform>
    ...
</Grid>