RecyclerView.setAdapter上的NullPointerException

Question

我正在尝试创建一个回收站视图，从flickr API中显示Json数据。

获取和解析Json正常工作。

显然我的适配器由于某种原因返回空指针。这是我的片段代码。

public class PhotoGalleryFragment extends Fragment {

private RecyclerView mPhotoRecyclerView;
private static final String TAG = "PhotoGalleryFragment";
private List<GalleryItem> mItems = new ArrayList<>();

public static PhotoGalleryFragment newInstance(){
    return new PhotoGalleryFragment();
}

@Override
public void onCreate(Bundle savedInstanceBundle){
    super.onCreate(savedInstanceBundle);
    setRetainInstance(true);
    new FetchItemsTask().execute();

}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceBundle){
    View v = inflater.inflate(R.layout.fragment_photo_gallery, container, false);
    RecyclerView mPhotoRecyclerView = (RecyclerView)v.findViewById(R.id.fragment_photo_gallery_recycler_view);
    mPhotoRecyclerView.setLayoutManager(new GridLayoutManager(getActivity(), 3));
    setupAdapter();
    return v;
}

private void setupAdapter(){
    if(isAdded()){
        mPhotoRecyclerView.setAdapter(new PhotoAdapter(mItems));
    }
}

private class FetchItemsTask extends AsyncTask<Void,Void,List<GalleryItem>>{
    @Override
    protected List<GalleryItem> doInBackground(Void... params){
        return new FlickrFetch().fetchItems();
    }

    @Override
    protected void onPostExecute(List<GalleryItem> items){
        mItems= items;
        setupAdapter();
    }
}

private class PhotoHolder extends RecyclerView.ViewHolder{

    private TextView mTitleTextView;

    public PhotoHolder(View itemView){
        super(itemView);
        mTitleTextView = (TextView) itemView;
    }

    public void bindGalleryItem(GalleryItem item){
        mTitleTextView.setText(item.toString());
    }
}

private class PhotoAdapter extends RecyclerView.Adapter<PhotoHolder>{

    private List<GalleryItem> mGalleryItems;

    public PhotoAdapter(List<GalleryItem> galleryItems){
        mGalleryItems = galleryItems;
    }

    @Override
    public PhotoHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        TextView textView = new TextView(getActivity());
        return new PhotoHolder(textView);
    }

    @Override
    public void onBindViewHolder(PhotoHolder photoHolder, int position) {
        GalleryItem galleryItem = mGalleryItems.get(position);
        photoHolder.bindGalleryItem(galleryItem);
    }

    @Override
    public int getItemCount() {
        return mGalleryItems.size();
    }
}

}

这是我的LogCat输出：

08-19 10:43:24.596 4920-4920/com.bignerdranch.android.photogallery E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                 Process: com.bignerdranch.android.photogallery, PID: 4920
                                                                                 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.bignerdranch.android.photogallery/com.bignerdranch.android.photogallery.PhotoGalleryActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setAdapter(android.support.v7.widget.RecyclerView$Adapter)' on a null object reference
                                                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
                                                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                                                                                     at android.app.ActivityThread.-wrap11(ActivityThread.java)
                                                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                                                                                     at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                     at android.os.Looper.loop(Looper.java:148)
                                                                                     at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                                                     at java.lang.reflect.Method.invoke(Native Method)
                                                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                                                                                  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setAdapter(android.support.v7.widget.RecyclerView$Adapter)' on a null object reference
                                                                                     at com.bignerdranch.android.photogallery.PhotoGalleryFragment.setupAdapter(PhotoGalleryFragment.java:51)
                                                                                     at com.bignerdranch.android.photogallery.PhotoGalleryFragment.onCreateView(PhotoGalleryFragment.java:45)
                                                                                     at android.support.v4.app.Fragment.performCreateView(Fragment.java:2074)
                                                                                     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)
                                                                                     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1286)
                                                                                     at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:758)
                                                                                     at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1671)
                                                                                     at android.support.v4.app.FragmentController.execPendingActions(FragmentController.java:388)
                                                                                     at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:619)
                                                                                     at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1237)
                                                                                     at android.app.Activity.performStart(Activity.java:6253)
                                                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2379)
                                                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                                                                                     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                                                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                                                                                     at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                                     at android.os.Looper.loop(Looper.java:148) 
                                                                                     at android.app.ActivityThread.main(ActivityThread.java:5417) 
                                                                                     at java.lang.reflect.Method.invoke(Native Method) 
                                                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                                                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)