Ошибка при переключение Tab

При переключении Tab, выкидывает ошибку:

E/AndroidRuntime: FATAL EXCEPTION: main
                  java.lang.NullPointerException
                      at android.support.v4.app.FragmentStatePagerAdapter.instantiateItem(FragmentStatePagerAdapter.java:120)
                      at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:1004)
                      at android.support.v4.view.ViewPager.populate(ViewPager.java:1218)
                      at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:663)
                      at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:625)
                      at android.support.v4.view.ViewPager.setCurrentItem(ViewPager.java:606)
                      at com.example.danilshik.testapplication.activities.SmartphoneActivity.onTabSelected(SmartphoneActivity.java:51)
                      at android.support.design.widget.TabLayout.dispatchTabSelected(TabLayout.java:1165)
                      at android.support.design.widget.TabLayout.selectTab(TabLayout.java:1158)
                      at android.support.design.widget.TabLayout.selectTab(TabLayout.java:1128)
                      at android.support.design.widget.TabLayout$Tab.select(TabLayout.java:1427)
                      at android.support.design.widget.TabLayout$TabView.performClick(TabLayout.java:1537)
                      at android.view.View$PerformClick.run(View.java:16966)
                      at android.os.Handler.handleCallback(Handler.java:615)
                      at android.os.Handler.dispatchMessage(Handler.java:92)
                      at android.os.Looper.loop(Looper.java:137)
                      at android.app.ActivityThread.main(ActivityThread.java:4745)
                      at java.lang.reflect.Method.invokeNative(Native Method)
                      at java.lang.reflect.Method.invoke(Method.java:511)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
                      at dalvik.system.NativeStart.main(Native Method)

Код, который взял с примера:

public class SmartphoneActivity extends AppCompatActivity implements TabLayout.OnTabSelectedListener {
    private TabLayout tabLayout;
    private ViewPager viewPager;
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_smartphone);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        tabLayout = (TabLayout) findViewById(R.id.tabLayout);
        tabLayout.addTab(tabLayout.newTab().setText("Главная"));
        tabLayout.addTab(tabLayout.newTab().setText("Дисплей"));
        tabLayout.addTab(tabLayout.newTab().setText("Структура"));
        tabLayout.addTab(tabLayout.newTab().setText("Оборудование"));
        tabLayout.addTab(tabLayout.newTab().setText("Камера"));
        tabLayout.addTab(tabLayout.newTab().setText("Передача данных"));
        tabLayout.addTab(tabLayout.newTab().setText("Аккумулятор"));

        tabLayout.setTabGravity(TabLayout.GRAVITY_FILL);
        viewPager = (ViewPager) findViewById(R.id.pager);
        SmartphonePager adapter = new SmartphonePager(getSupportFragmentManager(),tabLayout.getTabCount());
        viewPager.setAdapter(adapter);
        tabLayout.addOnTabSelectedListener(this);
    }
    @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) {
    }
}

UPD: SmartphonePager:

public class SmartphonePager extends FragmentStatePagerAdapter {
    int tabCount;
    public SmartphonePager(FragmentManager fm, int tabCount) {
        super(fm);
        this.tabCount = tabCount;
    }
    @Override
    public Fragment getItem(int position) {
        switch (position){
            case 0:
                SmartphoneMainTab smartphoneMainTab = new SmartphoneMainTab();
                return smartphoneMainTab;
            case 1:
                SmartphoneDisplayTab smartphoneDisplayTab = new SmartphoneDisplayTab();
                return smartphoneDisplayTab;
            default:
                return null;
        }
    }
    @Override
    public int getCount() {
        return tabCount;
    }