尝试调用虚拟方法'；java.lang.Class java.lang.Object.getClass（）；关于空对象引用

我正在使用Android studio。 当我尝试切换选项卡时，我的应用程序停止，并出现以下错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' 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:103)
       at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:1005)
       at android.support.v4.view.ViewPager.populate(ViewPager.java:1219)
       at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:665)
       at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:627)
       at android.support.v4.view.ViewPager.setCurrentItem(ViewPager.java:608)
       at android.support.design.widget.TabLayout$ViewPagerOnTabSelectedListener.onTabSelected(TabLayout.java:2169)
       at android.support.design.widget.TabLayout.dispatchTabSelected(TabLayout.java:1164)
       at android.support.design.widget.TabLayout.selectTab(TabLayout.java:1157)
       at android.support.design.widget.TabLayout.selectTab(TabLayout.java:1127)
       at android.support.design.widget.TabLayout$Tab.select(TabLayout.java:1426)
       at android.support.design.widget.TabLayout$TabView.performClick(TabLayout.java:1536)
       at android.view.View$PerformClick.run(View.java:21173)
       at android.os.Handler.handleCallback(Handler.java:739)
       at android.os.Handler.dispatchMessage(Handler.java:95)
       at android.os.Looper.loop(Looper.java:145)
       at android.app.ActivityThread.main(ActivityThread.java:6838)
       at java.lang.reflect.Method.invoke(Native Method)
       at java.lang.reflect.Method.invoke(Method.java:372)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)

我的代码，它将大部分信息存储在选项卡和片段上

请帮我找出我的错误

private SectionsPagerAdapter mSectionsPagerAdapter;

private ViewPager mViewPager;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.vault_screen);

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());


    mViewPager = (ViewPager) findViewById(R.id.container);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
    tabLayout.setupWithViewPager(mViewPager);

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });

}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_vault_screen, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
//deleted PlaceholderFragment class from here

/**
 * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
 * one of the sections/tabs/pages.
 */
public class SectionsPagerAdapter extends FragmentPagerAdapter {

    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {

        switch (position) {
            case 0:
                Tab1Contacts tab1 = new Tab1Contacts();
                return tab1;
            case 1:
                Tab2Chat tab2 = new Tab2Chat();
                return tab2;
            case 3:
                Tab3Online tab3 = new Tab3Online();
                return tab3;
            default:
        return null;
    }
    }

    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        switch (position) {
            case 0:
                return "Home";
            case 1:
                return "Private Browser";
            case 2:
                return "Notes";
        }
        return null;
    }

}

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我也犯了同样的错误，因为我试图通过标记或ID获取片段。但我必须通过创建新对象来获取片段，至少在没有对象的情况下

 Fragment fragment = new BlankFragment();

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您正在尝试处理空对象。我们需要您的代码片段，否则我们无法帮助您。我认为您的添加视图为空。提供更具体的代码..修复了当getCount（）返回错误号码时添加的代码有相同的错误。