public class MainActivity extends ActionBarActivity implements ActionBar.TabListener, UploadFragment.OnFragmentInteractionListener {
 
    SectionsPagerAdapter mSectionsPagerAdapter;
    ViewPager mViewPager;
 
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
 
        // Set up the action bar.
        final ActionBar actionBar = getSupportActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
 
        // Create the adapter that will return a fragment for each of the three
        // primary sections of the activity.
        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
 
        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mSectionsPagerAdapter);
 
        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++) {
 
            actionBar.addTab(
                    actionBar.newTab()
                            .setText(mSectionsPagerAdapter.getPageTitle(i))
                            .setTabListener(this));
        }
 
 
    }
 
 
    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 PlaceholderFragment (defined as a static inner class below).
            switch (position) {
                case 0:
                    return new ContainerFragment();
                case 1:
                    return null;
            }
            return null;
        }
 
        @Override
        public int getCount() {
            // Show 2 total pages.
            return 2;
        }
 
        @Override
        public CharSequence getPageTitle(int position) {
            Locale l = Locale.getDefault();
            switch (position) {
                case 0:
                    return getString(R.string.title_section1).toUpperCase(l);
                case 1:
                    return getString(R.string.title_section2).toUpperCase(l);
            }
            return null;
        }
    }
 
}
       
public class ContainerFragment extends Fragment {
 
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        /* Inflate the layout for this fragment */
        View view = inflater.inflate(R.layout.fragment_main, container, false);
 
        FragmentTransaction transaction = getChildFragmentManager().beginTransaction();
        /*
         * When this container fragment is created, we fill it with our first
         * "real" fragment
         */
        transaction.replace(R.id.frame, new UploadFragment());
        transaction.commit();
 
        return view;
    }
 
 
}
       
public class UploadFragment extends Fragment {
 
    /*Handler for the api call*/
    private AsyncHttpResponseHandler mConnectionUtil;
 
 
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_upload, container, false);
 
 
        /*The response handler declaration/definition */
        mConnectionUtil = new AsyncHttpResponseHandler() {
 
            @Override
            public void onSuccess(int i, Header[] headers, byte[] bytes) {
 
 
                    ResultFragment resultFrag = new ResultFragment();
                    FragmentTransaction ft = getChildFragmentManager().beginTransaction();
                    ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
                    ft.addToBackStack(null);
 
                    ft.replace(R.id.frame, resultFrag);
                    ft.commit();
 
 
            }
        };
 
        /*OnClickListener for the go/upload button*/
        mUploadButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                    /*Post to the api url with the params and the previously defined http request handler*/
                    AsyncHttpClient client = new AsyncHttpClient();
                    client.post("http://192.168.43.153:8000/form-submit", params, mConnectionUtil);
                }
            }
        });
 
        // Return fragment view
        return v;
    }
 
}
       
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
             android:layout_width="fill_parent"
             android:layout_height="fill_parent"
             android:id="@+id/frame"
        >
 
</FrameLayout>
       
java.lang.NullPointerException: Attempt to write to field 'android.support.v4.app.FragmentManagerImpl android.support.v4.app.Fragment.mFragmentManager' on a null object reference
            at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:394)
            at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:389)
            at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:99)
            at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:832)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:1048)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
            at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
            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 com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:327)
            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$DecorView.onMeasure(PhoneWindow.java:2939)
            at android.view.View.measure(View.java:16514)
            at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1912)
            at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1109)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1291)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:996)
            at android.view.ViewRootImpl$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:5141)
            at java.lang.reflect.Method.invoke(Native Method)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:799)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:615)