代码如下,执行时提示空指针异常,谁知道是什么原因?

执行后报的错:Object reference not set to an instance of an object.

XML文件:

<jobFlow>
  <unit>
    <unitID>00000</unitID>
    <name>BEGIN</name>
    <unitType>Terminator</unitType>
    <nextUnit>000003</nextUnit>
  </unit>

</jobFlow>

代码:

 string xmlPath = @"D:\a\xml\BIS100\MAIN_SECT.xml";

 XDocument myXDoc = XDocument.Load(xmlPath);
 XElement rootNode = myXDoc.Element("jobFlow");

下面注释的这部分代码执行是正常的
            //var query = from item in rootNode.Descendants("unit")
            //            select new
            //            {

           //                unitID = item.Element("unitID").Value,
            //                name = item.Element("name").Value,
            //                unitType = item.Element("unitType").Value,
            //                nextUnit = item.Element("nextUnit").Value,
            //            };

想直接转list<T>, 报错 Object reference not set to an instance of an object.

 List<JobFlow> jobFlows = (from item in rootNode.Descendants("unit")
                                     select new JobFlow
                                     {
                                         UnitID = item.Element("unitID").Value,
                                         Name = item.Element("name").Value,
                                         UnitType = item.Element("unitType").Value,
                                         NextUnit = item.Element("nextUnit").Value
                                     }).ToList<JobFlow>();

public class JobFlow
    {
        public string UnitID { get; set; }
        public string Name { get; set; }
        public string UnitType { get; set; }
        public string NextUnit { get; set; }

    }