新学WPF，有一个数据绑定的问题

Question

我做了一个窗体，有一个listview和textbox控件，现在是listview与本地xml文档相绑定，然后textbox显示listview选定项的某一列，listview的代码是这样的：

 <ListView x:Name="lstView" Grid.Row="3" VerticalAlignment="Top" Margin="3">
            <ListView.View>
                <GridView AllowsColumnReorder="True" ColumnHeaderToolTip="文件信息">
                    <GridViewColumn Header="音乐名称" DisplayMemberBinding="{Binding XPath=Title}"/>
                    <GridViewColumn Header="艺术家" DisplayMemberBinding="{Binding XPath=Artist}"/>
                    <GridViewColumn Header="发行时间" DisplayMemberBinding="{Binding XPath=Year}"/>
                    <GridViewColumn Header="唱片集" DisplayMemberBinding="{Binding XPath=Album}"/>
                    <GridViewColumn Header="文件地址" DisplayMemberBinding="{Binding XPath=@FullName}"/>
                </GridView>
            </ListView.View>
        </ListView>

textbox的代码：

<TextBox x:Name="tbxAddress" Text="{Binding ElementName=lstView, Path=SelectedItem,Converter={StaticResource pathConverter}}"/>

转换器如下：

 class MusicPathConverter:IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            XmlElement xElement =( XmlElement) value;
            return xElement.GetAttribute("FullName");
        }
        /// <summary>
        /// 未使用
        /// </summary>
        /// <param name="value"></param>
        /// <param name="targetType"></param>
        /// <param name="parameter"></param>
        /// <param name="culture"></param>
        /// <returns></returns>
        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

请教各位高手，为什么会报“未将对象引用设置到对象的实例”的异常呢？哪里出了问题?