Issue with reading nodes and its elements using XDocument in C#

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C#

I have an xml document like as shown below.

<?xml version="1.0" encoding="UTF-8"?>
   -<Projects>
     -<Project>
           <ProjectNo>111</ProjectNo>
           <Client/>
           <Responsible/>
           <Participants/>
           <Date>13-11-2015</Date>
     </Project>
    -<Transfers ID="1">
           <SampleID>1</SampleID>
           <SampleNo>222</SampleNo>
           <ChamberNo>p111</ChamberNo>
     </Transfers>
    -<Transfers ID="2">
           <SampleID>2</SampleID>
           <SampleNo>32</SampleNo>
           <ChamberNo>p311</ChamberNo>
     </Transfers>
    -<Transfers ID="3">
           <SampleID>3</SampleID>
           <SampleNo>42</SampleNo>
           <ChamberNo>p211</ChamberNo>
     </Transfers>
   </Projects>

I am trying to read nodes and its elements like this

XDocument xmlDocument = XDocument.Load(GlobalVariables.XmlPath + "\\"  + GlobalVariables.ProjectID + ".xml");

           var project = from p in xmlDocument.Descendants("Project")
                         select new Project
                         {

                             ProjectNo = p.Element("ProjectNo").Value,
                             Client = p.Element("Client").Value,
                             ProjectResponsible = p.Element("Responsible").Value,
                             Participants = p.Element("Participants").Value,
                             Date = Convert.ToDateTime(p.Element("Date").Value).Date,
                         };

           var Samples = (from s in xmlDocument.Descendants("Transfers")
                          select new Sample
                          {

                              SampleID = Convert.ToInt32(s.Element("SampleID").Value),
                              SampleNo = s.Element("SampleNo").Value,
                              ChamberNo = s.Element("ChamberNo").Value,
                          }

                         );
           int SamplesNo = Samples.Count();

           CreateWordDocument(project, Samples, SamplesNo);

I am able to read nodes and its elements at first time but if i edit values in xml file and try to read again then i am getting error "NullreferenceException was unhandled" Object reference not set to an instance of an object.

What might be the problem and how to rectify it?
Any suggestions?