Erro ao usar Fragment

/*
* Copyright (C) 2012 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.example.android.fragments;

import android.support.v4.*;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.Fragment;

public class MainActivity extends FragmentActivity 
implements HeadlinesFragment.OnHeadlineSelectedListener {

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.news_articles);

// Check whether the activity is using the layout version with
// the fragment_container FrameLayout. If so, we must add the first fragment
if (findViewById(R.id.fragment_container) != null) {

// However, if we're being restored from a previous state,
// then we don't need to do anything and should return or else
// we could end up with overlapping fragments.
if (savedInstanceState != null) {
return;
}

// Create an instance of ExampleFragment
HeadlinesFragment firstFragment = new HeadlinesFragment();

// In case this activity was started with special instructions from an Intent,
// pass the Intent's extras to the fragment as arguments
firstFragment.setArguments(getIntent().getExtras() );

// Add the fragment to the 'fragment_container' FrameLayout
getSupportFragmentManager().beginTransaction()
.add(R.id.fragment_container, firstFragment).commit();
}
}

public void onArticleSelected(int position) {
// The user selected the headline of an article from the HeadlinesFragment

// Capture the article fragment from the activity layout
ArticleFragment articleFrag = (ArticleFragment)
getSupportFragmentManager().findFragmentById(R.id. article_fragment);

if (articleFrag != null) {
// If article frag is available, we're in two-pane layout...

// Call a method in the ArticleFragment to update its content
articleFrag.updateArticleView(position);

} else {
// If the frag is not available, we're in the one-pane layout and must swap frags...

// Create fragment and give it an argument for the selected article
ArticleFragment newFragment = new ArticleFragment();
Bundle args = new Bundle();
args.putInt(ArticleFragment.ARG_POSITION, position);
newFragment.setArguments(args);
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack so the user can navigate back
transaction.replace(R.id.fragment_container, newFragment);
transaction.addToBackStack(null);

// Commit the transaction
transaction.commit();
}
}
}