根据2个Texboxes binded do滑块中的值在TexBox中执行计算

Question

我有这段代码：

    <Grid HorizontalAlignment="Center" VerticalAlignment="Center">
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="Auto" />
        <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition Width="300*" />
        <ColumnDefinition Width="Auto" />
    </Grid.ColumnDefinitions>

    <Label x:Name="labelMultiplicador" Grid.Row="0" Grid.Column="0" Content="Multiplicador:" FontWeight="Bold" Margin="5" HorizontalAlignment="Left" VerticalAlignment="Center" Width="100" />
    <Slider x:Name="sliderMultiplicador" Grid.Row="0" Grid.Column="1" Orientation="Horizontal" SmallChange="1" IsSnapToTickEnabled="True" Minimum="1" VerticalAlignment="Center" Height="1" MinHeight="30" HorizontalContentAlignment="Stretch" VerticalContentAlignment="Center" MinWidth="300" Margin="5" TickPlacement="TopLeft" />
    <TextBox x:Name="textBoxMultiplicador" Grid.Row="0" Grid.Column="2" Width="50" Text="{Binding ElementName=sliderMultiplicador,Path=Value}" HorizontalAlignment="Center" VerticalAlignment="Center" Padding="1,0,0,0" Margin="5" HorizontalContentAlignment="Center" VerticalContentAlignment="Center" MinHeight="30" />

    <Label x:Name="labelMultiplicando" Grid.Row="1" Grid.Column="0" Content="Multiplicando:" FontWeight="Bold" Margin="5" HorizontalAlignment="Left" VerticalAlignment="Center" Width="100" />
    <Slider x:Name="sliderMultiplicando" Grid.Row="1" Grid.Column="1" Orientation="Horizontal" SmallChange="1" IsSnapToTickEnabled="True" Minimum="1" VerticalAlignment="Center" MinHeight="30" HorizontalContentAlignment="Stretch" VerticalContentAlignment="Center" MinWidth="300" Margin="5" Height="1" TickPlacement="TopLeft" />
    <TextBox x:Name="textBoxMultiplicando" Grid.Row="1" Grid.Column="2" Width="50" Text="{Binding ElementName=sliderMultiplicando,Path=Value}" HorizontalAlignment="Center" VerticalAlignment="Center" Padding="1,0,0,0" Margin="5" HorizontalContentAlignment="Center" VerticalContentAlignment="Center" MinHeight="30" />

    <Label x:Name="labelProduto" Grid.Row="2" Grid.Column="0" Content="Produto:" FontWeight="Bold" Margin="5" HorizontalAlignment="Left" VerticalAlignment="Center" Width="100" />
    <TextBox x:Name="textBoxProduto" Grid.Row="2" Grid.Column="2" Width="50" Text="" HorizontalAlignment="Right" VerticalAlignment="Center" Padding="1,0,0,0" Margin="5" HorizontalContentAlignment="Right" VerticalContentAlignment="Center" MinHeight="30" />
</Grid>

我想使用textBoxMultiplicador和textBoxMultiplicando中的值计算产品（textBoxProduto）。 我是WPF的新手，所以这可能是一个“愚蠢”的问题!!!

如果我尝试使用textBoxMultiplicador_TextChanged事件计算产品，例如，e得到一个“'System.NullReferenceException'”。 在这种情况下，最好的方法是什么？ 感谢

此处的完整示例：https://www.dropbox.com/s/e8w9t59vjhnqjbb/tabuada.zip?dl=0