title: NavigationService.Navigate Throwing NullReferenceException WP7?
body: <p>Just Downloaded the WP7 SDK yestarday - And I'm new to Silver light - and Application development in general. </p>
  
  <p>I'm just simply trying to Direct my user to a different page, but I have a NullRefernceException Error occur on a Line Of Code. 
  The Stupid thing - the code above it is exactly the same - yet doesn't throw the error? </p>
  
  <pre><code>if (myISO.DirectoryExists("Logs") &amp;&amp; myISO.DirectoryExists("DataStore"))
  {
       NavigationService.Navigate(new Uri("Sign-in.xaml", UriKind.Relative));
  }
  else
  {
       NavigationService.Navigate(new Uri("Welcome.xaml", UriKind.Relative));
       //Above Line Causes Exception^
  }
  </code></pre>
  
  <p>Also I get an NullRefernceException in App.xaml.cs - which I haven't even changed - it WAS working - now it doesn't : </p>
  
  <pre><code>PhoneApplicationService.Current.UserIdleDetectionMode = IdleDetectionMode.Disabled;
  </code></pre>