Importé un proyecto de muestra de Android llamado HorizontalPaging en Android Studio y funciona bien cuando lo ejecuto. Pero cuando copié el código en mi código,Obtengo una excepción de puntero NULL en getActionBar ().

He estado leyendo sobre estos problemas todo el día, pero no puedo hacer que funcione. Intenté copiar todo el código de MainActivity de la muestra en mi proyecto, pero sin mejoras, así que supongo que el problema está en algún otro archivo como manifiesto, estilos, etc.

MainActivity.java copiado del proyecto de muestra

import android.app.ActionBar;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import java.util.Locale;


public class View_Tabs extends FragmentActivity implements ActionBar.TabListener {


SectionsPagerAdapter mSectionsPagerAdapter;


ViewPager mViewPager;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // Load the UI from res/layout/activity_main.xml
    setContentView(R.layout.view_tabs);


    final ActionBar actionBar = getActionBar();
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);


    // Create the adapter that will return a fragment for each of the three primary sections
    // of the app.
    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    // When swiping between different sections, select the corresponding tab. We can also use
    // ActionBar.Tab#select() to do this if we have a reference to the Tab.

    mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
        @Override
        public void onPageSelected(int position) {
            actionBar.setSelectedNavigationItem(position);
        }
    });



    // For each of the sections in the app, add a tab to the action bar.
    for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
        // Create a tab with text corresponding to the page title defined by the adapter. Also
        // specify this Activity object, which implements the TabListener interface, as the
        // callback (listener) for when this tab is selected.
        actionBar.addTab(
                actionBar.newTab()
                        .setText(mSectionsPagerAdapter.getPageTitle(i))
                        .setTabListener(this));
    }

}


@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
    // When the given tab is selected, tell the ViewPager to switch to the corresponding page.
    mViewPager.setCurrentItem(tab.getPosition());
}



@Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}


@Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}


public class SectionsPagerAdapter extends FragmentPagerAdapter {


    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }



    @Override
    public Fragment getItem(int position) {
        // getItem is called to instantiate the fragment for the given page.
        // Return a DummySectionFragment (defined as a static inner class
        // below) with the page number as its lone argument.
        Fragment fragment = new DummySectionFragment();
        Bundle args = new Bundle();
        args.putInt(DummySectionFragment.ARG_SECTION_NUMBER, position + 1);
        fragment.setArguments(args);
        return fragment;
    }


    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }


    @Override
    public CharSequence getPageTitle(int position) {
        Locale l = Locale.getDefault();
        switch (position) {
            case 0:
                return getString(R.string.tab_list).toUpperCase(l);
            case 1:
                return getString(R.string.tab_map).toUpperCase(l);
            case 2:
                return getString(R.string.tab_list).toUpperCase(l);
        }
        return null;
    }

}

/**
 * A dummy fragment representing a section of the app, but that simply displays dummy text.
 * This would be replaced with your application's content.
 */
public static class DummySectionFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this
     * fragment.
     */
    public static final String ARG_SECTION_NUMBER = "section_number";

    public DummySectionFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.list_tab_fragment, container, false);
        TextView dummyTextView = (TextView) rootView.findViewById(R.id.section_label);
        dummyTextView.setText(Integer.toString(getArguments().getInt(ARG_SECTION_NUMBER)));
        return rootView;
    }
}



}

mi manifiesto

<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"/>

<!-- TABS: While ViewPager will work on API 4 or above, tabs require an ActionBar. ActionBar is only
 available in API 11 or above. -->
<!-- Min/target SDK versions (<uses-sdk>) managed by build.gradle -->

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >



    <meta-data android:name="com.google.android.gms.version"
        android:value="@integer/google_play_services_version" />

    <!--
    <meta-data android:name="com.google.android.gms.version"
        android:value="@integer/google_play_services_version" />
    -->

    <meta-data
        android:name="com.google.android.maps.v2.API_KEY"
        android:value="xxxxx"/>

    <activity
        android:name=".MainActivity"
        android:label="@string/app_name"
        android:screenOrientation="portrait"
        >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

.... 

    <activity
        android:name=".View_Tabs"
        android:label="@string/app_name"
        android:screenOrientation="portrait"
        >
    </activity>
</application>

my styles.xml

<resources>

    <!-- Base application theme. -->
    <!-- BEFORE <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">  -->
    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
        <!-- Customize your theme here. -->
        <item name="android:windowActionBar">true</item>
    </style>

    <style name="Theme.Base" parent="android:Theme.Light" />




</resources>

mi gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 21
    buildToolsVersion "21.1.2"

    defaultConfig {
        applicationId "xxxx"
        minSdkVersion 14
        targetSdkVersion 21
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:21.0.3'
    compile 'com.google.android.gms:play-services:6.5.87'
    //compile 'com.google.android.gms:play-services:7.0.0'
    compile files('lib/gson-2.3.jar')
}