title: Error:Value cannot be null. Parameter name: source
body: <p>I'm very new to programming and I keep on getting this error eventhough I checked for nulls...I can't figure out why it is still giving me this error. 
  This is my code in my ProjectDetailsViewModel:</p>
  
  <pre><code> public List&lt;string&gt; SelectedServices
          {
              get
              {
                  var services =
                      Project.Services?.Select(x =&gt; x.ProjServiceCode).ToList();
                  var service = Project.ProjectPricings.Where(x=&gt;x.ProjectId == Project.Id &amp;&amp; !string.IsNullOrWhiteSpace(x.ServiceCode))
                      .Select(x =&gt; x.ServiceCode)
                      .ToList();
                  var serviceList = new List&lt;string&gt;();
                  if (services != null) serviceList.AddRange(services);
                  serviceList.AddRange(service);
                  return serviceList ;
              }
          }
  </code></pre>
  
  <p>and here I'm using it in the View:</p>
  
  <pre><code> &lt;tr class="control-oddrow"&gt;
          &lt;td class="control-label"&gt;
              &lt;label class="dcl-control-label required"&gt;Addt'l Services:&lt;/label&gt;
          &lt;/td&gt;
          &lt;td class="spacer-column"&gt;&lt;/td&gt;
          &lt;td class="control-input"&gt;
              @{ var services = Model.SelectedServices ?? new List&lt;string&gt;() ; }
  
              @Html.ListBoxFor(m =&gt; services, new MultiSelectList(Model.ProjectServices, "ServiceCode", "ServiceCode", services))
          &lt;/td&gt;
      &lt;/tr&gt;
  </code></pre>
  
  <p>everytime I want to get the details of a project that the services in null, it throws this error. i would really appreciate some help. Thanks in advance.</p>