title: Null reference exception when try to get link by class in HtmlAgilityPack
body: <p>I have asp.net mvc application and html page which I parse using HtmlAgilityPack, but when I try looping my elements I have next error in my foreach: <code>Object reference not set to an instance of an object</code>. My code is next. Does anybody know where is my mistake? I'm new with using htmlagilitypack.</p>
  
  <p><strong>Part of HTML:</strong></p>
  
  <pre><code>&lt;li class="b-serp-item i-bem" onclick="return {&amp;quot;b-serp-item&amp;quot;:{}}"&gt;
    &lt;i class="b-serp-item__favicon" style="background-position: 0 -0px"&gt;&lt;/i&gt;
    &lt;h2 class="b-serp-item__title"&gt;
      &lt;b class="b-serp-item__number"&gt;1&lt;/b&gt;
      &lt;a class="b-serp-item__title-link" href="http://googlescraping.com/google-scraper.php"&gt;Google&lt;/a&gt;
    &lt;/h2&gt;
  &lt;/li&gt;
  </code></pre>
  
  <p><strong>CODE</strong></p>
  
  <pre><code>DateTime dt = DateTime.Now;
  string dtf = String.Format("{0:u}", dt);
  string wp = "page" + dtf + ".html";
  HtmlDocument HD = new HtmlDocument();
  HD.Load(wp);
  string output = "";
  foreach (HtmlNode node in HD.DocumentNode.SelectNodes("//a[@class='b-serp-item__title-link']"))
  {
      output += node.GetAttributeValue("href", null) + " ";
  }
  </code></pre>
  
  <p>Html output I was shared in google drive:
  <a href="https://drive.google.com/file/d/0B3-m-r5Ce0gOSTlzUGlTT1VBb00/edit?usp=sharing" rel="nofollow">https://drive.google.com/file/d/0B3-m-r5Ce0gOSTlzUGlTT1VBb00/edit?usp=sharing</a></p>