我想创建带有PagerView的片段，但运行以下代码会返回一个IllegalStateException：

源代码：

public class HomeFragment extends Fragment {

private TabLayout tabs;
private ClassesFragment classesFragment;
private ExamsFragment examsFragment;


@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    classesFragment = new ClassesFragment();
    examsFragment = new ExamsFragment();
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    View v = inflater.inflate(R.layout.fragment_main_classes, container, false);
    Toolbar toolbar = getActivity().findViewById(R.id.toolbar);

    ViewPager viewPager = new ViewPager(getContext());
    viewPager.setLayoutParams(new Toolbar.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
    setupViewPager(viewPager);
    toolbar.addView(viewPager);

    tabs = new TabLayout(getContext());
    tabs.setLayoutParams(new Toolbar.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT));
    toolbar.addView(tabs);
    tabs.setupWithViewPager(viewPager);
    return v;
}

// Add Fragments to Tabs
private void setupViewPager(ViewPager viewPager) {
    PagerAdapter adapter = new PagerAdapter(getChildFragmentManager());
    adapter.addFragment("Занятия", ExamsFragment.newInstance());
    viewPager.setAdapter(adapter);
}

private class PagerAdapter extends FragmentPagerAdapter {
    private List<Fragment> fragments = new ArrayList<>();
    private List<String> titles = new ArrayList<>();

    public PagerAdapter(FragmentManager fm) {
        super(fm);
        fragments.add(ExamsFragment.newInstance());
        titles.add("new");
    }

    @Override
    public Fragment getItem(int position) {
        return fragments.get(position);
    }

    public void addFragment(String title, Fragment fragment)
    {
        titles.add(title);
        fragments.add(fragment);
    }

    @Override
    public int getCount() {
        return fragments.size();
    }

    @Override
    public CharSequence getPageTitle(int position) {
        return titles.get(position);
    }
}

IllegalStateException：

进程：com.company.user.blabla,pid：22256 java.lang.illegalstate异常：带有适配器的ViewPager com.company.user.blabla.fragments.homeFragment$pageradapter@ca3cef6需要一个视图id（位于android.support.v4.app.fragmentpageradapter.81)（位于android.support.v4.view.ViewPager.populate（位于ViewPager.java:1119)（位于android.support.v4.view.ViewPager.populate（位于：685）在Android.support.design.widget.AppBarLayout.OnMeasure(AppBarLayout.java:223)在Android.view.view.measure(view.java:21998)在Android.view.view.view.measureChild(viewgroup.java:6580)在Android.support.design.widget.coordinatorLayout.OnMeasureChild(coordinatorLayout.java:713)在1998）在Android.View.ViewGroup.MeasureChildWithMargins（ViewGroup.java:6580)在Android.Widget.LinearLayout.MeasureChildBeForelayout（LinearLayout.java:1514)在Android.Widget.LinearLayout.MeasureVertical（LinearLayout.java:806)在Android.Widget.LinearLayout.OnMeasure(LinearLayout.java:685)在nal.policy.decorview.onMeasure(decorview.java:721)在Android.view.view.measure(view.java:21998)在Android.view.viewrootimpl.performMeasure(viewrootimpl.java:2410)在Android.view.viewrootimpl.measurehierarchy(viewrootimpl.java:1498)在Android.view.viewrootimpl.performtraversal(viewrootimpl.java:1751)在

我对安卓系统没有太多经验。搜索没有用或者我搜索不正确。我能做些什么来解决这个问题？