Android TabLayout inside Fragment

After looking into yours code, I found that you forget to return a fragment in getItem method of PagerAdapter that you overrided from the FragmentStatePagerAdapter and there is no break statement in each case due to which every time a null value was returned.

To doing so you need to create a another new fragment for each page in ViewPager or you can create one new fragment that could use in all page of ViewPager.

You can't use the same recyclerview into the tabs of view pager that you have defined in parent fragment.

activity_fragment_one.xml

 <android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context=".NewFragment">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.design.widget.TabLayout
            android:id="@+id/tabs"
            android:layout_width="match_parent"
            app:layout_behavior="@string/appbar_scrolling_view_behavior"
            app:tabTextAppearance="?android:attr/textAppearanceMedium"
            app:tabTextColor="#ffffff"
            android:layout_height="?android:actionBarSize"
            app:tabMode="fixed"
            app:tabGravity="fill"/>

    </android.support.design.widget.AppBarLayout>

    <android.support.v4.view.ViewPager
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"
        android:background="@android:color/white"/>
</android.support.design.widget.CoordinatorLayout>

I have fixed your fragment and created a new Fragment that you can use in ViewPager Pages.

public class NewFragment extends Fragment {

private RecyclerView mRecyclerView;


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.activity_fragment_one, container, false);
    TabLayout tabLayout = (TabLayout) view.findViewById(R.id.tabs);
    tabLayout.addTab(tabLayout.newTab().setText("A"));
    tabLayout.addTab(tabLayout.newTab().setText("B"));
    tabLayout.addTab(tabLayout.newTab().setText("C"));
    final ViewPager viewPager = (ViewPager) view.findViewById(R.id.viewpager);
   // mRecyclerView = (RecyclerView) view.findViewById(R.id.recycler_view);
    LinearLayoutManager mLayoutManager = new LinearLayoutManager(getActivity());
    mLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
    //mRecyclerView.setLayoutManager(mLayoutManager);
    viewPager.setAdapter(new PagerAdapter(getFragmentManager(), tabLayout.getTabCount()));
    viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));
    tabLayout.setupWithViewPager(viewPager);
    tabLayout.setTabMode(TabLayout.MODE_SCROLLABLE);
    tabLayout.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {

        @Override
        public void onTabSelected(TabLayout.Tab tab) {
            viewPager.setCurrentItem(tab.getPosition());
        }

        @Override
        public void onTabUnselected(TabLayout.Tab tab) {

        }

        @Override
        public void onTabReselected(TabLayout.Tab tab) {

        }
    });

    return view;
}

public class PagerAdapter extends FragmentStatePagerAdapter {
    int mNumOfTabs;

    public PagerAdapter(FragmentManager fm, int NumOfTabs) {
        super(fm);
        this.mNumOfTabs = NumOfTabs;
    }


    @Override
    public Fragment getItem(int position) {

        switch (position) {
            case 0:
              return new FragmentTab();
            case 1:
                return new FragmentTab();
            case 2:
                return new FragmentTab();

            default:
                return null;
        }
    }

    @Override
    public int getCount() {
        return mNumOfTabs;
    }
}
}

FragmentTab for each Page in View Pager (or you can create different fragment for each page according to yours need).

public class FragmentTab extends Fragment {
RecyclerView mRecyclerView;

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_tab, container, false);

    mRecyclerView = (RecyclerView) view.findViewById(R.id.recycler_view);
    LinearLayoutManager mLayoutManager = new LinearLayoutManager(getActivity());
    mLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
    mRecyclerView.setLayoutManager(mLayoutManager);
    RecyclerView.Adapter mAdapter = new AdapterOne();
    mRecyclerView.setAdapter(mAdapter);
    return view;
}
}

fragment_tab.xml

<?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.v7.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        />


</LinearLayout>

Use getChildFragmentManager() instead of getSupportFragmentManager()

i suggest using getChildFragmentManager instead of getFragmentManager in

viewPager.setAdapter(new PagerAdapter (getFragmentManager(), tabLayout.getTabCount()));

Without the error logs, it's difficult to say where and why do you get the NullPointerException.

The first thing that caught my attention is that you didn't link your TabLayout and ViewPager. Use the following code after you set the adapter for the ViewPager:

// setUpWithViewPager doesn't work without using a Runnable interface.
// Support library bug, maybe?
tabLayout.post(new Runnable() {
    @Override
    public void run() {
        tabLayout.setupWithViewPager(viewPager);
    }
});

I'm not sure if this is the problem, but give it a try :)