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) {

   }

}