ViewPager+Fragment设置监听器后程序崩溃

Question

本来写的ViewPager+Fragment好好地，移动页卡也没问题，但是我加上监听事件以后，再去滑动页卡程序就崩溃强退了。下面是MainActivity代码：package com.example.myapplication;import android.graphics.Color;import android.support.v4.app.Fragment;import android.support.v4.app.FragmentActivity;import android.support.v4.view.PagerTabStrip;import android.support.v4.view.ViewPager;import android.os.Bundle;import android.view.View;import android.widget.Toast;import java.util.ArrayList;import java.util.List;public class MainActivity extends FragmentActivity implements ViewPager.OnPageChangeListener {    private List<View> viewList;    private ViewPager pager;    private List<String> titleList;    private PagerTabStrip tabStrip;    private List<Fragment>fragmentList;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        //初始化        viewList =new ArrayList<>();        titleList =new ArrayList<>();        fragmentList =new ArrayList<>();        //加载Fragment        fragmentList.add(new MyFragment1());        fragmentList.add(new MyFragment2());        fragmentList.add(new MyFragment3());        fragmentList.add(new MyFragment4());        //为页卡加载标题        titleList.add("第一页");        titleList.add("第二页");        titleList.add("第三页");        titleList.add("第四页");        tabStrip= (PagerTabStrip) findViewById(R.id.tab);        //设置标题属性        tabStrip.setBackgroundColor(Color.BLACK);        tabStrip.setTextColor(Color.WHITE);        tabStrip.setDrawFullUnderline(false);        tabStrip.setTabIndicatorColor(Color.RED);        pager= (ViewPager) findViewById(R.id.pager);        //加载页卡        View view1=View.inflate(this,R.layout.view1,null);        View view2=View.inflate(this,R.layout.view2,null);        View view3=View.inflate(this,R.layout.view3,null);        View view4=View.inflate(this,R.layout.view4,null);        viewList.add(view1);        viewList.add(view2);        viewList.add(view3);        viewList.add(view4);        //设置适配器        MyPagerAdapter pagerAdapter =new MyPagerAdapter(viewList,titleList);        MyFragmentPagerAdapter pagerAdapter1 =new MyFragmentPagerAdapter(getSupportFragmentManager(),fragmentList,titleList);        //加载适配器        //pager.setAdapter(pagerAdapter);        pager.setAdapter(pagerAdapter1);        //加载监听器        pager.addOnPageChangeListener(this);    }    @Override    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {    }    @Override    public void onPageSelected(int position) {        Toast.makeText(this,position,Toast.LENGTH_SHORT).show();    }    @Override    public void onPageScrollStateChanged(int state) {    }}