Gridview在进行活动时不会在手机上显示图像

Question

FATAL EXCEPTION: main
                                                         Process: a2day.a2day, PID: 28475
                                                         java.lang.RuntimeException: Unable to start activity ComponentInfo{a2day.a2day/a2day.a2day.Explore}: java.lang.NullPointerException: Attempt to get length of null array
                                                             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2947)
                                                             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3008)
                                                             at android.app.ActivityThread.-wrap14(ActivityThread.java)
                                                             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1650)
                                                             at android.os.Handler.dispatchMessage(Handler.java:102)
                                                             at android.os.Looper.loop(Looper.java:154)
                                                             at android.app.ActivityThread.main(ActivityThread.java:6688)
                                                             at java.lang.reflect.Method.invoke(Native Method)
                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468)
                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358)
                                                          Caused by: java.lang.NullPointerException: Attempt to get length of null array
                                                             at a2day.a2day.Explore.imageReader(Explore.java:75)
                                                             at a2day.a2day.Explore.onCreate(Explore.java:28)
                                                             at android.app.Activity.performCreate(Activity.java:6912)
                                                             at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1126)
                                                             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2900)
                                                             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3008) 
                                                             at android.app.ActivityThread.-wrap14(ActivityThread.java) 
                                                             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1650) 
                                                             at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                             at android.os.Looper.loop(Looper.java:154) 
                                                             at android.app.ActivityThread.main(ActivityThread.java:6688) 
                                                             at java.lang.reflect.Method.invoke(Native Method) 
                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468) 
                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358) 

以上是我试图从我的目录加载图片时接近的错误...下面我正在添加java类，我必须尝试在Android设备上加载图像。

import android.net.Uri;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.ImageView;

public class ViewImage extends AppCompatActivity {

ImageView iv2;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.view_image);

    String f = getIntent().getStringExtra("img");

    iv2 = (ImageView) findViewById(R.id.imageView2);
    iv2.setImageURI(Uri.parse(f));
}

}

public class Explore extends ActionBarActivity {

GridView gv;
ArrayList<File> list;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_explore);

    list = imageReader(Environment.getExternalStorageDirectory());

    gv = (GridView) findViewById(R.id.gridView);
    gv.setAdapter(new GridAdapter());

    gv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            startActivity(new Intent(getApplicationContext(), ViewImage.class).putExtra("img", list.get(position)));
        }
    });

}

private class GridAdapter extends BaseAdapter {

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

    @Override
    public Object getItem(int position) {
        return list.get(position);
    }

    @Override
    public long getItemId(int position) {
        return 0;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        convertView = getLayoutInflater().inflate(R.layout.my_grid, parent ,false);
        ImageView iv = (ImageView) convertView.findViewById(R.id.singleGrid);

        iv.setImageURI(Uri.parse(getItem(position).toString()));

        return convertView;
    }
}

ArrayList<File> imageReader(File root){
    ArrayList<File> a = new ArrayList<>();

    File[] files = root.listFiles();
    for(int i =0; i < files.length; i++) {
        if(files[i].isDirectory()) {
            a.addAll(imageReader(files[i]));
        } else {
            if(files[i].getName().endsWith(".jpg")) {
                a.add(files[i]);
            }
        }
    }
    return a;
}

}

有人可以帮我解决这个问题吗？我不知道它有什么问题，它给我的压力太大，因为我没有时间完成这项工作，因为我还有其他需要完成的工作。