title: NullreferenceException when using NavigationService
body: <p>I'm encountering a problem with my WP7 app:</p>
  
  <p>I'm trying to make a login page prompt only when the app can't detect any stored username or password values; however, I keep running into a NullReferenceException when I try to navigate to my login page.</p>
  
  <p>My code looks like this, and it's in the contructor:</p>
  
  <pre><code>if (!checkLogin())
      {
          NavigationService.Navigate(new Uri("/LoginPage.xaml", UriKind.Relative));
      }
  </code></pre>
  
  <p>And <code>checkLogin</code> is just a function that returns either true or false depending or not the isolated storage settings are set correctly.</p>
  
  <p>Anybody have any suggestions?</p>
  
  <p>Thanks.</p>