调用函数viewpager时片段为null

Question

My Fragment课程：

onCreatedView:
parentActivity = (MainActivity) getActivity();
     parentView = inflater.inflate(R.layout.fragmentla, container, false);
 mFragmentAdapter = new myFragmentAdapter(parentActivity.getSupportFragmentManager(), parentView.getContext());

        // Get the ViewPager and set it's PagerAdapter so that it can display items
        ViewPager viewPager = (ViewPager) parentView.findViewById(R.id.viewpager);
        viewPager.setAdapter(mFragmentAdapter);

        // Give the TabLayout the ViewPager
        TabLayout tabLayout = (TabLayout) parentView.findViewById(R.id.tablayout);
        tabLayout.setupWithViewPager(viewPager);

这是我的片段（蓝色边框）。我的片段包含工具栏，tablayout和viewpager。

My FragmentAdapter类：

public class myFragmentAdapter extends FragmentStatePagerAdapter {
    final int PAGE_COUNT = 2;
    private String tabTitles[] = new String[] { "ITEM1", "ITEM2" };
    private Context context;

    public myFragmentAdapter(FragmentManager fm, Context context) {
        super(fm);
        this.context = context;
    }

    @Override
    public int getCount() {
        return 2;
    }

    @Override
    public Fragment getItem(int position) {
        if(position == 0) {
            return FirstFragment.newInstance(position + 1);
        } else {
            return SecondFragment.newInstance(position + 1);
        }
    }

    @Override
    public void notifyDataSetChanged() {
        super.notifyDataSetChanged();
    }

    @Override
    public CharSequence getPageTitle(int position) {
        // Generate title based on item position
        return tabTitles[position];
    }

}

这是我的FirstFragment类：

public static FirstFragment newInstance(int page) {
        Bundle args = new Bundle();
        args.putInt(ARG_PAGE, page);
        FirstFragment fragment = new FirstFragment();
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mPage = getArguments().getInt(ARG_PAGE);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        parentView = inflater.inflate(R.layout.fragment_first_newest, container, false);
sometext = (TextView) parentView.findViewById(R.id.wi_my_text);
    return parentView;
    }

所以，我的问题是：每当我按下工具栏中的菜单按钮时，textview应该改变，但是这个返回null导致“onCreatedView”不再被调用。我在Fragment类中试过这个：

 public boolean onOptionsItemSelected(MenuItem item){
...
 Fragment mFragment = mFragmentAdapter.getItem(0);

                    if(mFragment instanceof FirstFragment){
                        ((FirstFragment) mFragment).OnButtonPressed();
                    }
}

然后在我的FirstFragment上，但仍然得到null异常：

public void OnButtonPressed() {
        someText.setText("its working");
        createLocationRequest();
    }

更新：我的XML：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

                <TextView
                    android:layout_width="wrap_content"
                    android:id="@+id/wi_my_text"
                    android:layout_height="24dp"
                    android:layout_gravity="center"
                    android:gravity="center"
                    android:textStyle="bold"
                    android:paddingEnd="5dp"
                    android:text="texttest"/>
</LinearLayout>