无法在ViewPager中设置FragmentStatePagerAdapter

我想通过方法在ViewPager中设置FragmentStatePagerAdapter的子类的对象

viewPager.setAdapter(myPagerAdapter);

但是我得到如下错误

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.app.Fragment.setMenuVisibility(boolean)' on a null object reference
                                                       at android.support.v4.app.FragmentStatePagerAdapter.instantiateItem(FragmentStatePagerAdapter.java:120)
                                                       at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:1006)
                                                       at android.support.v4.view.ViewPager.populate(ViewPager.java:1154)
                                                       at android.support.v4.view.ViewPager.populate(ViewPager.java:1088)
                                                       at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1614)

请帮助我解决此问题。

该代码段是

    public class MainActivity extends AppCompatActivity {

        Toolbar toolbar;
        TabLayout tabLayout;
        ViewPager viewPager;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            toolbar = (Toolbar)findViewById(R.id.toolbar);
            toolbar.setTitle("My App");
            toolbar.setSubtitle("this is my test");
            tabLayout = (TabLayout)findViewById(R.id.tablayout);
            tabLayout.addTab(tabLayout.newTab().setText("Left"));
            tabLayout.addTab(tabLayout.newTab().setText("Cent"));
            tabLayout.addTab(tabLayout.newTab().setText("Right"));

            viewPager = (ViewPager)findViewById(R.id.viewpage);

            MyPagerAdapter myPagerAdapter = new MyPagerAdapter(
                    getSupportFragmentManager(),
                    tabLayout.getTabCount()
            );
            viewPager.setAdapter(myPagerAdapter);
            //viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));
        }
    }

public class MyPagerAdapter extends FragmentStatePagerAdapter {
    int tabCount;
    public MyPagerAdapter(FragmentManager fm, int tabCount) {
        super(fm);
        this.tabCount = tabCount;
    }

    @Override
    public Fragment getItem(int position) {
        switch (position){
            case 1:
                Fragment1 fragment1 = new Fragment1();
                return fragment1;
            case 2:
                Fragment2 fragment2 = new Fragment2();
                return fragment2;
            case 3:
                Fragment3 fragment3 = new Fragment3();
                return fragment3;
            default:
                return null;
        }
    }

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