Android : NullpointerException using ViewPager

I want to have Tabs in my Application and to do so I use the ViewPager but I get this logcat:

04-17 22:03:31.291  10545-10545/de.gestureanywhere E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: de.gestureanywhere, PID: 10545
java.lang.NullPointerException
        at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:416)
        at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:411)
        at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:99)
        at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:837)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:987)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:919)
        at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1441)
        at android.view.View.measure(View.java:16514)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
        at android.view.View.measure(View.java:16514)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
        at android.view.View.measure(View.java:16514)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
        at android.support.v7.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:453)
        at android.view.View.measure(View.java:16514)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
        at android.view.View.measure(View.java:16514)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
        at android.view.View.measure(View.java:16514)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2556)
        at android.view.View.measure(View.java:16514)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1912)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1109)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1291)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:996)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5600)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
        at android.view.Choreographer.doCallbacks(Choreographer.java:574)
        at android.view.Choreographer.doFrame(Choreographer.java:544)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
        at android.os.Handler.handleCallback(Handler.java:733)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5034)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:611)
        at dalvik.system.NativeStart.main(Native Method)

Here is my code:

public class Vorschlag extends ActionBarActivity implements
    ActionBar.TabListener {

private ViewPager viewPager;
private TabsAdapter mAdapter;
private ActionBar actionBar;

// Tab titles
private String[] tabs = { "Neuer Vorschlag", "Vorschläge" };

@Override
protected void onCreate(Bundle savedInstanceState) {
    requestWindowFeature(Window.FEATURE_ACTION_BAR);
    super.onCreate(savedInstanceState);



    setContentView(R.layout.vorschlag_layout);

    ActionBar actionBar = getSupportActionBar();

    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_LIST);

    actionBar.setDisplayShowTitleEnabled(true);

    mAdapter = new TabsAdapter(getSupportFragmentManager());

    // Initilization
    viewPager = (ViewPager) findViewById(R.id.pager);

   // actionBar = getApplicationContext().getS


    viewPager.setAdapter(mAdapter);



    // Adding Tabs
    for (String tab_name : tabs) {
        actionBar.addTab(actionBar.newTab().setText(tab_name)
                .setTabListener(this));
    }
}







@Override
public void onTabSelected(ActionBar.Tab tab, android.support.v4.app.FragmentTransaction fragmentTransaction) {

}

@Override
public void onTabUnselected(ActionBar.Tab tab, android.support.v4.app.FragmentTransaction fragmentTransaction) {

}

@Override
public void onTabReselected(ActionBar.Tab tab, android.support.v4.app.FragmentTransaction fragmentTransaction) {

}

}

this is the layout file:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.v4.view.ViewPager
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    </android.support.v4.view.ViewPager>



</LinearLayout>

And here is my Adapter class:

import android.support.v4.app.FragmentManager;

import android.support.v4.app.FragmentPagerAdapter;

/** * Created by Viktor on 17.04.2015. */ public class TabsAdapter extends FragmentPagerAdapter {

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

@Override
public android.support.v4.app.Fragment getItem(int index) {

    switch (index) {
        case 0:
            // Top Rated fragment activity
            // return new TopRatedFragment();
            break;
        case 1:
            // Games fragment activity
            // return new GamesFragment();
            new VorhandeneVorschlaege();
            break;

    }

    return null;
}

@Override
public int getCount() {
    // get item count - equal to number of tabs
    return 2;
}

}