Display Image in Listview with AsyncTask

I am trying to display images in a listview with AsyncTask but I am getting an error. Saying...

Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference

This is error is happening on

public LoadImage(ImageView imv) {
        this.imv = imv;
        this.path = imv.getTag().toString();
}

And

    LoadImage loadImage = new LoadImage(imageView);
    Bitmap bitmap = loadImage.doInBackground();

Here is LoadImage.java

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.os.AsyncTask;
import android.os.Environment;
import android.view.View;
import android.widget.ImageView;

import java.io.File;

/**
 * Created by Yudii on 27/05/2016.
 */
class LoadImage extends AsyncTask<Object, Void, Bitmap>{

    private ImageView imv;
    private String path;

    public LoadImage(ImageView imv) {
        this.imv = imv;
        this.path = imv.getTag().toString();
    }

    @Override
    protected Bitmap doInBackground(Object... params) {

        Bitmap bitmap = null;
        File file = new File(Environment.getExtealStorageDirectory()
                .getAbsolutePath() + path);

        if (file.exists()) {
            bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
        }

        retu bitmap;
    }
    @Override
    protected void onPostExecute(Bitmap result) {


        if (!imv.getTag().toString().equals(path)) {
               /* The path is not same. This means that this
                  image view is handled by some other async task.
                  We don't do anything and retu. */
          //  retu;
        }




        if(result != null && imv != null){
            imv.setVisibility(View.VISIBLE);
            imv.setImageBitmap(result);
        }else{
            imv.setVisibility(View.GONE);
        }
    }

}

And here is CustomList.java

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.drawable.BitmapDrawable;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.TextView;

import java.util.ArrayList;

public class CustomList extends ArrayAdapter<String>{

    private final Activity context;
    private final Integer[] imageId;

    public CustomList(Activity context,
                      String[] web, Integer[] imageId) {
        super(context, R.layout.list_single, web);
        this.context = context;
        this.imageId = imageId;

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

        LayoutInflater inflater = context.getLayoutInflater();
        View rowView= inflater.inflate(R.layout.list_single, null, true);

        ImageView imageView = (ImageView) rowView.findViewById(R.id.img);


        LoadImage loadImage = new LoadImage(imageView);
        Bitmap bitmap = loadImage.doInBackground();


        imageView.setImageResource(imageId[position]);

        retu rowView;

    }
}

You need to change loadImage.execute(); instead of Bitmap bitmap = loadImage.doInBackground();