title: findFragmentByTag() returns null
body: <p><strong>UPDATE:</strong> 
  Just to clarify, <code>show()</code> is an <code>android.app.DialogFragment</code>'s method that I didn't overwrote:</p>
  
  <pre><code>public void show(FragmentManager manager, String tag) {
      mDismissed = false;
      mShownByMe = true;
      FragmentTransaction ft = manager.beginTransaction();
      ft.add(this, tag);
      ft.commit();
  }
  </code></pre>
  
  <p>We call the following, where <code>MyDialog</code> extends <code>DialogFragment</code>:</p>
  
  <pre><code>    MyDialog dialog = new MyDialog();
      dialog.show(getFragmentManager(), MyDialog.TAG);
  
      Fragment f = getFragmentManager().findFragmentByTag(MyDialog.TAG);
  </code></pre>
  
  <p>But <code>f</code> is always <code>null</code>. Why?</p>