android：ViewPager抛出空指针

Question

我有一个包含多个片段的应用程序。 我的第一个片段是HomeFragment，我正在使用viewpager和recycler视图。

 public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            // Inflate the layout for this fragment
            View view = inflater.inflate(R.layout.fragment_home, container, false);
            ButterKnife.bind(this, view);
            ((AppCompatActivity) getActivity()).getSupportActionBar().setTitle(Constant.HOME_TITLE);
            // view Pager
            mPager = (ViewPager) view.findViewById(R.id.pager);

            // recycler view
            rv_transactions.setHasFixedSize(true);
            LinearLayoutManager mLayoutmanager = new LinearLayoutManager(getContext());
            rv_transactions.setLayoutManager(mLayoutmanager);
    }

在HomeFragment的onStart中，我正在使用asyncTask进行网络调用。在asynctask的compltion我有一个回调onTaskComplete（）我在哪里设置我的列表和寻呼机

    public void onTaskComplete(JSONObject response){

// some jsonparsing logic, then

                    TransactionRecyclerViewAdapter adapter = new TransactionRecyclerViewAdapter(cardInfoAndTransactionArrayList.get(0).getTransactions());
                    Log.d("rv_adapter",""+cardInfoAndTransactionArrayList.get(0).getTransactions());
                    rv_transactions.setAdapter(adapter);

                    mPagerAdapter = new MultipleCardPagerAdapter(getChildFragmentManager(), cardInfoAndTransactionArrayList);

                    Log.d("ArrayList size",""+cardInfoAndTransactionArrayList.size());
                    Log.d("mPagerAdapter",""+mPagerAdapter);
                    Log.d("mPager",""+mPager);
                    mPager.setAdapter(mPagerAdapter);
                    mPager.addOnPageChangeListener(this);
    }

然后有一个片段名称IgFragment从我回到HomeFragment

android.support.v4.app.FragmentManager fManager = getActivity().getSupportFragmentManager();    
Boolean isPoped = fManager.popBackStackImmediate(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);
                    HomeFragment homeFragment = HomeFragment.newInstance(mobileNumber, "");
                    FragmentTransaction transaction = fManager.beginTransaction();
                                        transaction.replace(R.id.fragment_container, homeFragment);
                    transaction.commit();

app的流程就像HomeFragment - ＆gt;第1片段 - ＆gt;第二片段 - ＆gt; IGFragment - ＆gt; HomeFragment和我正在添加1＆amp; 2 to addToBackStack（null）有后退按钮导航但在IGFragment我正在清理后台堆栈，因为当我在homeFragment时我不想要任何后退导航行为但是清除backstack使viewPager抛出nullpointer异常

logcat output (before exception thrown)
ArrayList size: 2
/mPagerAdapter: com.example.worldline.adapters.MultipleCardPagerAdapter@36efdd11

/mPager: android.support.v4.view.ViewPager{5a27e76 VFED.... ......ID 0,0-1080,486 #7f0f00f8 app:id/pager}


logcat Output (Exception thrown at  mPager.setAdapter(mPagerAdapter);)
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference

如果我不清除堆栈一切正常但HomeFragment在按下后退按钮时显示多个片段（但是没有异常来自viewpager）但是如果我清除backstack它会抛出空指针，即使arraylist的大小为2（logact输出）在将适配器设置为viewpager之前打印）

MultipleCardPagerAdapter代码

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter; 
import android.support.v4.view.PagerAdapter;

import com.example.worldline.pojo.CardInfoAndTransactionsPojo;
import com.example.worldline.r_cug.MultipleCardViewPagerFragment;

import java.util.ArrayList;

/**
 * Created by sunny.birman on 28-10-2016.
 */

public class MultipleCardPagerAdapter extends FragmentStatePagerAdapter {

    private ArrayList<CardInfoAndTransactionsPojo> cardDetails;
    private int cardListLength;

    public MultipleCardPagerAdapter(FragmentManager fm, ArrayList<CardInfoAndTransactionsPojo> list) {
        super(fm);
        this.cardDetails= list;
        cardListLength = list.size();
    }

    @Override
    public Fragment getItem(int position) {
        return MultipleCardViewPagerFragment.newInstance(cardDetails.get(position).getCardNumber(),cardDetails.get(position).getCardBalance(),position,cardListLength);
    }

    @Override
    public int getCount() {
        return cardDetails.size();
    }

    @Override
    public int getItemPosition(Object object) {
        return PagerAdapter.POSITION_NONE;
    }


}