从活动访问片段时出错

Question

我尝试从活动中更改Fragment-UI，但它不起作用。 我在HeaderSrollingViewBehaviour.java中得到一个NullPointerException 是AppBarLayout.java的一部分。
这只发生在我从updateUIonFragmentOverview（）调用fragmentOverview.updateUI（）时。

public class MainActivity extends AppCompatActivity {

    private Toolbar toolbar;
    private TabLayout tabLayout;
    private ViewPager viewPager;

    FragmentOverview fragmentOverview;

    String toastOnUIThreadMessage;

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

        //initialisiert das Feld monitoringActivity im Application-Objekt
        //dieses weiß dann, das die Activity gestartet wurde
        ((BillingSystemApplication) this.getApplicationContext()).setMonitoringActivity(this);

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

        getSupportActionBar().setDisplayHomeAsUpEnabled(true);

        viewPager = (ViewPager) findViewById(R.id.viewpager);
        setupViewPager(viewPager);

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

        //initialisieren des FragmentOverviews
        fragmentOverview = new FragmentOverview();
        tabLayout.setupWithViewPager(viewPager);


        updateUIonFragmentOverview();
    }

    private void setupViewPager(ViewPager viewPager) {
        ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());
        adapter.addFragment(fragmentOverview, "Übersicht");
        adapter.addFragment(new FragmentOverview(), "Teilstrecken");
        viewPager.setAdapter(adapter);
    }

    class ViewPagerAdapter extends FragmentPagerAdapter {
        private final List<Fragment> mFragmentList = new ArrayList<>();
        private final List<String> mFragmentTitleList = new ArrayList<>();

        public ViewPagerAdapter(FragmentManager manager) {
            super(manager);
        }

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

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

        public void addFragment(Fragment fragment, String title) {
            mFragmentList.add(fragment);
            mFragmentTitleList.add(title);
        }

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


    public void ToastOnUIThread(String message)
    {
        toastOnUIThreadMessage = message;
        MainActivity.this.runOnUiThread(new Runnable() {
            @Override
            public void run()
            {
                Toast.makeText(MainActivity.this, toastOnUIThreadMessage, Toast.LENGTH_LONG).show();
            }
        });
    }



    public void updateUIonFragmentOverview()
    {
        fragmentOverview.updateUI();
    }
}

logcat的

FATAL EXCEPTION: main
Process: com.cyberlandgo.felix.workt20, PID: 14147
 java.lang.NullPointerException
        at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:418)
        at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:413)
        at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:103)
        at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:1003)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:1151)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:1085)
        at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1611)
        at android.view.View.measure(View.java:17331)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5376)
        at android.support.design.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:671)
        at android.support.design.widget.HeaderScrollingViewBehavior.onMeasureChild(HeaderScrollingViewBehavior.java:90)
        at android.support.design.widget.AppBarLayout$ScrollingViewBehavior.onMeasureChild(AppBarLayout.java:1319)
        at android.support.design.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:736)
        at android.view.View.measure(View.java:17331)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5376)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
        at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:135)
        at android.view.View.measure(View.java:17331)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5376)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1410)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
        at android.view.View.measure(View.java:17331)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5376)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
        at android.view.View.measure(View.java:17331)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5376)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1410)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
        at android.view.View.measure(View.java:17331)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5376)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2548)
        at android.view.View.measure(View.java:17331)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2279)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1393)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1592)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1251)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6540)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:813)
        at android.view.Choreographer.doCallbacks(Choreographer.java:613)
        at android.view.Choreographer.doFrame(Choreographer.java:583)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:799)
        at android.os.Handler.handleCallback(Handler.java:733)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:146)
        at android.app.ActivityThread.main(ActivityThread.java:5593)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099)
        at dalvik.system.NativeStart.main(Native Method)

我只想访问片段UI并进行更改 我希望你能帮助我！