title: I get "Possible null reference return" warning, although result is declared maybe null
body: <p>I am new to C#. I do not get why I still get this warning when I have marked the return type with &quot;?&quot;. See screen shot attached.</p>
  <pre><code>    public Item? GetItem(Guid id)
      {
          return items.Where(item =&gt; item.Id == id).SingleOrDefault();
      }
  </code></pre>
  <p><a href="https://i.stack.imgur.com/tM9wn.png" rel="nofollow noreferrer">Code and warning</a></p>