页面跳转后bottomNavigationView监听器显示空指针 程序闪退

package com.example.test3;



import android.annotation.SuppressLint;

import android.os.Build;

import android.os.Bundle;

import android.support.annotation.NonNull;

import android.support.annotation.RequiresApi;

import android.support.design.widget.BottomNavigationView;

import android.support.v4.view.ViewPager;

import android.support.v7.app.AppCompatActivity;

import android.view.MenuItem;

import android.view.MotionEvent;

import android.view.View;









public class Main2Activity extends AppCompatActivity {



    private ViewPager viewPager;

    private MenuItem menuItem;

    private BottomNavigationView bottomNavigationView;



    @RequiresApi(api = Build.VERSION_CODES.KITKAT)

    @SuppressLint("ClickableViewAccessibility")

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        getSupportActionBar().hide();// 隐藏ActionBar

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

        bottomNavigationView = (BottomNavigationView) findViewById(R.id.main_bottom_nav_view);





        bottomNavigationView.setOnNavigationItemSelectedListener(

                new BottomNavigationView.OnNavigationItemSelectedListener() {

                    @Override

                    public boolean onNavigationItemSelected(@NonNull MenuItem item) {

                        switch (item.getItemId()) {

                            case R.id.btm_nav_item1:

                                viewPager.setCurrentItem(0);

                                break;

                            case R.id.btm_nav_item2:

                                viewPager.setCurrentItem(1);

                                break;



                        }

                        return false;

                    }

                });



        viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {

            @Override

            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {



            }



            @Override

            public void onPageSelected(int position) {

                if (menuItem != null) {

                    menuItem.setChecked(false);

                } else {

                    bottomNavigationView.getMenu().getItem(0).setChecked(false);

                }

                menuItem = bottomNavigationView.getMenu().getItem(position);

                menuItem.setChecked(true);

            }



            @Override

            public void onPageScrollStateChanged(int state) {

            }

        });



// 禁止ViewPager滑动

        viewPager.setOnTouchListener(new View.OnTouchListener() {

            @Override

            public boolean onTouch(View v, MotionEvent event) {

                return true;

            }

        });



        setupViewPager(viewPager);

    }



    private void setupViewPager(ViewPager viewPager) {

        ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());



        adapter.addFragment(new oneFragment());



        adapter.addFragment(new twoFragment());

        viewPager.setAdapter(adapter);

    }

}