代码中的空引用异常

我在“_attr.append(xmlnode.attributes[”name“]);”上获取nullreferenceexception错误。

namespace SMAS
{
class Profiles
{
    private XmlTextReader _profReader;
    private XmlDocument _profDoc;

    private const string Url = "http://localhost/teamprofiles.xml";
    private const string XPath = "/teams/team-profile";

    public XmlNodeList Teams{ get; private set; }
    private XmlAttributeCollection _attr;

    public ArrayList Team { get; private set; }

    public void GetTeams()
    {
        _profReader = new XmlTextReader(Url);
        _profDoc = new XmlDocument();

        _profDoc.Load(_profReader);
        Teams = _profDoc.SelectNodes(XPath);

        foreach (XmlNode xmlNode in Teams)
        {
            _attr.Append(xmlNode.Attributes["name"]);
        }
    }
}
}

teamprofiles.xml文件看起来像

  <teams>
    <team-profile name="Australia">
      <stats type="Test">
        <span>1877-2010</span>
        <matches>721</matches>
        <won>339</won>
        <lost>186</lost>
        <tied>2</tied>
        <draw>194</draw>
        <percentage>47.01</percentage>
      </stats>
      <stats type="Twenty20">
        <span>2005-2010</span>
        <matches>32</matches>
        <won>18</won>
        <lost>12</lost>
        <tied>1</tied>
        <draw>1</draw>
        <percentage>59.67</percentage>
      </stats>
    </team-profile>
    <team-profile name="Bangladesh">
      <stats type="Test">
        <span>2000-2010</span>
        <matches>66</matches>
        <won>3</won>
        <lost>57</lost>
        <tied>0</tied>
        <draw>6</draw>
        <percentage>4.54</percentage>
      </stats>
    </team-profile>
 </teams>

我试图在一个数组中提取所有团队的名字。然后我将提取所有团队的所有统计信息,并将它们显示在我的应用程序中。你能帮我解决那个空引用异常吗?