title: GeckoFX - implement search function - how to use nsIWebBrowserFind interface
body: <p>Could anyone perhaps tell me how should I use the nsIWebBrowserFind interface in GeckoFX to find strings on a webpage?</p>
  
  <p>I tried the following code, but this throws me an ArgumentNullException - parameter cannot be null (pUnk).</p>
  
  <p>I have no idea what this means, I have never used interfaces before.</p>
  
  <pre><code>        GeckoWebBrowser browser = getCurrentBrowser();
  
          nsIWebBrowserFind finder = browser.GetInterface&lt;nsIWebBrowserFind&gt;();
  
          finder.SetSearchStringAttribute(searchBox1.Text);
          finder.FindNext();
  </code></pre>
  
  <p>I have also tried </p>
  
  <pre><code>    nsIWebBrowserFind finder = Gecko.Xpcom.GetInterface&lt;nsIWebBrowserFind&gt;(browser);
  </code></pre>
  
  <p>With the same results:(</p>
  
  <p>Please help:)</p>
  
  <p>Thanks!</p>