初学者，请大神帮忙：未将对象引用设置到对象的实例

         <Grid x:Name="LayoutRoot" Background="White">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="260"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="120"/>
            <RowDefinition Height="120"/>
        </Grid.RowDefinitions>
        <!-- StrokeThickness为边框宽度 -->
        <Rectangle Grid.Row="0" Grid.Column="1" x:Name="PreviewColor" Fill="#FF6600" Margin="10" Stroke="#666666"
                   StrokeThickness="2"/>
        <StackPanel Grid.Row="1" Grid.Column="1">
            <TextBlock FontSize="12">Color
            </TextBlock>
            <TextBox x:Name="HexColor" Width="160" Height="30" Text="#FF6600" Margin="10,5" FontSize="11"/>
        </StackPanel>
        <StackPanel Grid.Row="0" Grid.Column="0" Grid.RowSpan="2" VerticalAlignment="Center">
            <TextBlock Text="Alpha" FontSize="12" Margin="10,15,0,0"/>
            <Slider x:Name="AlphaSlider" Margin="20,0,10,0" Maximum="255" Value="255"
                    ValueChanged="RedSlider_ValueChanged"  />
            <TextBlock Text="Red" FontSize="12" Margin="10,15,0,0"/>
            <Slider x:Name="RedSlider" Margin="20,0,10,0" Maximum="255" Value="255"
                    ValueChanged="RedSlider_ValueChanged"/>
            <TextBlock Text="Green" FontSize="12" Margin="10,15,0,0"/>
            <Slider x:Name="GreenSlider" Margin="20,0,10,0" Maximum="255" Value="255"
                    ValueChanged="RedSlider_ValueChanged"/>
            <TextBlock Text="Blue" FontSize="12" Margin="10,15,0,0"/>
            <Slider x:Name="BlueSlider" Margin="20,0,10,0" Maximum="255" Value="255"
                    ValueChanged="RedSlider_ValueChanged"/>
        </StackPanel>
    </Grid>
提示问题：未将对象引用设置到对象的实例
        private void RedSlider_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
        {
            Color color=Color.FromArgb((byte)AlphaSlider.Value,(byte)RedSlider.Value,
                (byte)GreenSlider.Value,(byte)BlueSlider.Value);
            PreviewColor.Fill = new SolidColorBrush(color);
            HexColor.Text = color.ToString();
        }为什么AlphaSlider,RedSlider,GreenSlider,BlueSlider会为空呢