title: NavigationView get/find header layout
body: <p>In my NavigationView I have a header layout with id 'viewId' with active buttons. To setup those buttons, I do the following in activity's <code>onPostCreate</code>:</p>
  
  <pre><code>final View panel = findViewById(R.id.viewId);
  panel.setOnClickListener(new View.OnClickListener() {
  ... setup goes here ...
  });
  </code></pre>
  
  <p>With new version android support library, (<strong>23.1.0</strong>), the view can't be found, it returns null. With previous versions it worked well. Is it a bug or am I using this feature wrong? If so, how to access header layout and add behavior to it?</p>