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);
    }
}这是第二个acticity，在这上面添加了fragment，达到滑动的目的，在这之前还有一个登录界面，之前没有设置登录界面直接就是这个acticity，代码一样的可以运行 ，添加了一个登录界面之后 点击登录按钮就闪退，然后报错 bottomNavigationView这里出现空指针，各位大神能看看吗