title: SharePoint lookup field returning null value
body: <p>I am using CSOM to retrieve data from sharePoint online.
  I need to get data from a document lib. here is syntax which I used to retrieve data.</p>
  
  <pre><code>List list = clientContext.Web.Lists.GetByTitle("Required Documents");
  if (list != null)
  {
  CamlQuery caml = new CamlQuery();
  caml.ViewXml = @"&lt;View&gt;
                    &lt;Query&gt;
                      &lt;Where&gt;
                         &lt;Eq&gt;
                           &lt;FieldRef Name='PONo' /&gt;
                           &lt;Value Type='Lookup'&gt;" + poNo + @"&lt;/Value&gt;
                          &lt;/Eq&gt;
                      &lt;/Where&gt;
                   &lt;/Query&gt;                                            
                  &lt;/View&gt;";                                               
  
  ListItemCollection items = list.GetItems(caml);
  clientContext.Load&lt;ListItemCollection&gt;(items);
  clientContext.ExecuteQuery();
  </code></pre>
  
  <p>here PONo is a lookup for another list item.
  So I tried to get value as below, but it returns null.</p>
  
  <pre><code>var itm = item.FieldValues["PONo"] as FieldUserValue;
  </code></pre>
  
  <p>when try like this,</p>
  
  <pre><code>var itm = item.FieldValues["PONo"];
  </code></pre>
  
  <p>It returns needful value. what would be the problem?</p>