title: mahapps metro showMessageAsync error: " Object reference not set to an instance of an object"
body: <p>I have this method</p>
  
  <pre><code>    public async void setState(int state)
      {
          switch (state)
          {
              default:
                  await this.ShowMessageAsync("Title", "Message");
                  break;
          }
      }
  </code></pre>
  
  <p>which I call from the cuntructor of the MetroWindow. But I get this error:</p>
  
  <pre><code>An exception of type 'System.NullReferenceException' occurred in MahApps.Metro.dll but was not handled in user code
  
  Additional information: Object reference not set to an instance of an object.
  </code></pre>
  
  <p>On the line</p>
  
  <pre><code>await this.ShowMessageAsync("Title", "Message");
  </code></pre>
  
  <p>How do I fix this?</p>