title: findFragmentById always returns null
body: <p>I'm defining an ID for my fragment in the xml layout:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:id="@+id/test_fragment"
  ...
  </code></pre>
  
  <p>Then I add this fragment in the activity's onCreate method:</p>
  
  <pre><code>MyFragment myFragment = new MyFragment();
  fragmentTransaction.add(R.id.fragment_container, myFragment);
  fragmentTransaction.commit();
  </code></pre>
  
  <p>This is all working fine. Replacing fragments and is also working.</p>
  
  <p>Later I'm trying to retrieve this fragment by its ID in one of the activity's methods:</p>
  
  <pre><code>MyFragment myFragment = (MyFragment) getFragmentManager().findFragmentById(R.id.test_fragment);
  </code></pre>
  
  <p>Doing so leads to myFragment being <code>null</code>. Always.</p>
  
  <p>When I try to do the same with tags instead of IDs I can retrieve the fragment by its tag without any problems:</p>
  
  <pre><code>MyFragment myFragment = new MyFragment();
  fragmentTransaction.add(R.id.fragment_container, myFragment, "testfragment");
  fragmentTransaction.commit();
  </code></pre>
  
  <p>...</p>
  
  <pre><code>MyFragment myFragment = (MyFragment) getFragmentManager().findFragmentByTag("testfragment");
  </code></pre>
  
  <p>Why can't findFragmentById find the fragment, but findFragmentByTag does so? Am I missing something?</p>