title: Activity's onDestroy / Fragment's onDestroyView set Null practices
body: <p>I am reading <strong>ListFragment</strong> source code and I see this implementation:</p>
  
  <pre><code>ListAdapter mAdapter;
  ListView mList;
  View mEmptyView;
  TextView mStandardEmptyView;
  View mProgressContainer;
  View mListContainer;
  CharSequence mEmptyText;
  boolean mListShown;
  
  /**
   * Detach from list view.
   */
  @Override
  public void onDestroyView() {
      mHandler.removeCallbacks(mRequestFocus);
      mList = null;
      mListShown = false;
      mEmptyView = mProgressContainer = mListContainer = null;
      mStandardEmptyView = null;
      super.onDestroyView();
  }
  </code></pre>
  
  <p>In this function, Google developers set Null to all view fields that declared in ListFragment and remove callback 'mRequestFocus'.</p>
  
  <p>In <strong>ListActivity</strong> source code. Google developers implemented like below:</p>
  
  <pre><code>protected ListAdapter mAdapter;
  protected ListView mList;
  
  private Handler mHandler = new Handler();
  
  
  @Override
  protected void onDestroy() {
      mHandler.removeCallbacks(mRequestFocus);
      super.onDestroy();
  }
  </code></pre>
  
  <p><strong>I didn't see Google developers set Null to mList on onDestroy of ListActivity as they did for ListFragment class.</strong></p>
  
  <p><strong>My question is</strong></p>
  
  <ol>
  <li><p>Why google developers didnot set Null to mList in onDestroy of ListActivity? Any reasons?</p></li>
  <li><p>Do we need to set Null to all View fields in Activity's onDestroy and Fragment's onDestroyView?</p></li>
  </ol>
  
  <p><strong>3. Any practices for set Null in these two functions: Activity's onDestroy and Fragment's onDestroyView?</strong></p>
  
  <p>Thank you for your ideas!</p>