从活动中的片段获取视图

Question

我试图从Fragment中获取动态添加到Activity的视图。

MainActivity.java

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

    ...

    if (savedInstanceState == null) {
        // on first time display view for first nav item
        displayView(0);

    }

    myDrawerList.setOnItemClickListener(new DrawerItemClickListener());
    handleIntent(getIntent());

}

private class DrawerItemClickListener implements ListView.OnItemClickListener {
    @Override
    public void onItemClick(
        AdapterView<?> parent, View view, int position, long id
    ) {
        // display view for selected nav drawer item
        displayView(position);
    }               
}

private void displayView(int position) {
    // update the main content by replacing fragments
    Fragment fragment = null;
    String userToken = UserTokenStorageFactory.instance().getStorage().get();
    switch (position) {
    case 0:
        fragment = new AllCategories();
        break;
    case 1:
        fragment = new Subscription();
        break;
    case 2:
        if( userToken != null && !userToken.equals( "" ) ){
            Intent myIntent1 = new Intent(this, PrivateOffice.class);
            this.startActivity(myIntent1);
            break;
        }
        else {
            Intent myIntent1 = new Intent(this, LogIn.class);
            this.startActivity(myIntent1);
            break;
        }
    case 3:
        if( userToken != null && !userToken.equals( "" ) ){
            Intent myIntent1 = new Intent(this, Create_action.class);
            this.startActivity(myIntent1);
            break;
        }
        else {
            Intent myIntent1 = new Intent(this, LogIn.class);
            this.startActivity(myIntent1);
            break;
        }
    case 4:
        AlertDialog dialog = onCreateDialog(DIALOG_CHOOSE_CATEGORY);
        dialog.show();
        break;
    default:
        break;
    }

    if (fragment != null) {
        android.app.FragmentManager fragmentManager = getFragmentManager();
        fTrans = fragmentManager.beginTransaction();
        fTrans.replace(R.id.content_frame, fragment);
        fTrans.commit();

        // update selected item and title, then close the drawer
        myDrawerList.setItemChecked(position, true);
        myDrawerList.setSelection(position);
        setTitle(viewsNames[position]);
        myDrawerLayout.closeDrawer(mDrawer);
    } else {
        // error in creating fragment
        Log.e("MainActivity", "Error in creating fragment");
    }
}

我需要从AllCategories.java的布局中获取视图（LinearLayout listv）。我尝试将findView（）与findViewById一起使用，但它返回NullPointerException。我该怎么办？