I am getting NullReferenceException error on "_attr.Append(xmlNode.Attributes["name"]);".

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"]);
        }
    }
}
}

the teamprofiles.xml file looks like

  <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>

I am trying to extract names of all teams in an ArrayList. Then i'll extract all stats of all teams and display them in my application. Can you please help me about that null reference exception?