Getting imageview inside ListView Adapter

Question

I made an adapter for a ListView where I can get the id and the name, but I can't get the image.

Here is some code samples:

private class MyCustomAdapter extends BaseAdapter {
    public ArrayList<Exercise> exercises ;

    public MyCustomAdapter(ArrayList<Exercise>  exercises) {
        this.exercises=exercises;
    }


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

    @Override
    public String getItem(int position) {
        return null;
    }

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

    @Override
    public View getView(int position, View convertView, ViewGroup parent)
    {
        LayoutInflater mInflater = getLayoutInflater();
        View myView = mInflater.inflate(R.layout.detail_item, null);

        final   Exercise s = exercises.get(position);

        TextView txtid=( TextView)myView.findViewById(R.id.tv_id);
        txtid.setText(""+s.id);

        TextView txtname=( TextView)myView.findViewById(R.id.tv_name);
        txtname.setText(s.name);

        ImageView img = (ImageView) myView.findViewById(R.id.img_id);
        img.setImageResource(s.img);




        return myView;
    }

}

The error is here:

(ImageView img = (ImageView) myView.findViewById(R.id.img_id); img.setImageResource(s.img);)

The Exception:

::: Process: com.example.simo.gym_programs, PID: 11826 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageResource(int)' on a null object reference at com.example.simo.gym_programs.Detail_Activity$MyCustomAdapter.getView(Detail_Activity.java:73)