title: Xamarin.Forms: Start an activity using DependencyService
body: <p>In my Xamarin.Forms.Page I have a DependencyService that must activate an activity in my .Droid project. How do I activate the Activity?
  I've tried </p>
  
  <pre><code>StartActivity(typeof(FacebookActivity));
  </code></pre>
  
  <p>but I get </p>
  
  <blockquote>
    <p>Java.Lang.NullPointerException: Exception of type
    'Java.Lang.NullPointerException' was thrown.</p>
  </blockquote>
  
  <p>What do I have to initialize? </p>