NullPointException on making an arraylist of objects in android

I'm new in android programming . I'm writing an android program which has two tabs: detailstab in which user fill a form and hit save button. listtab in which display what user insert in edittexts control in details tab. to implement this i made this classes:

detail java class :

import android.widget.RadioButton;

import android.widget.TextView;

public class detail {

public   TextView  tvName;
public   TextView  tvAdr;
public   TextView  tvNote;
public  RadioButton rb_0;
public  RadioButton rb_1;
public  RadioButton rb_2;
public int radioValue;
//public  final TextView  tvRadioValue=null;




}

details activity class in which i make an arraylist and add objects from detail:

import java.util.ArrayList;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.TextView;

public class details extends Activity {

public  ArrayList<detail> detaillist=new ArrayList<detail>();
public EditText etName;
public EditText etAdr;
public  EditText etNote;
public  RadioGroup rg;
public  RadioButton rb_0;
public  RadioButton rb_1;
public  RadioButton rb_2;

 static details detailsActivity;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    detailsActivity=this;
    setContentView(R.layout.details_tab);



    Button btnSave=(Button) findViewById(R.id.button1);
    //define an arraylist to add detail object to it 

    etName=(EditText) findViewById(R.id.editText1);
    etAdr=(EditText) findViewById(R.id.editText2);
    etNote=(EditText) findViewById(R.id.editText3);
     rg=(RadioGroup) findViewById(R.id.radioGroup1);
     rb_0=(RadioButton) findViewById(R.id.radio0);
     rb_1=(RadioButton) findViewById(R.id.radio1);
     rb_2=(RadioButton) findViewById(R.id.radio2);

    btnSave.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View arg0) {

            final detail d=new detail();
            makeDetailList(d);






            //clear textboxes
            clear(etName);
            clear(etAdr);
            clear(etNote);
            rg.check(R.id.radio0);




        }
    });

}

public void clear(View v) {
     EditText edittext=new EditText(null);
     edittext.setText("");

 } 
 public ArrayList<detail> makeDetailList(detail d){



        d.tvName.setText(etName.getText());
        d.tvAdr.setText(etAdr.getText());
        //d.tvNote.setText(etNote.getText());

        if(rb_0.isSelected())
        {
            d.radioValue=0;
            //d.tvRadioValue.setText(rb_0.getText());
        }
        else if (rb_1.isSelected())
        {
            d.radioValue=1;
            //d.tvRadioValue.setText(rb_1.getText());
        }
        else if (rb_1.isSelected())
        {
            d.radioValue=2;
            //d.tvRadioValue.setText(rb_2.getText());
        }


        detaillist.add(d);

    return detaillist;

 }


 public static details getInstance(){
       return   detailsActivity;
     }



}

and finally lists activity class this class contains a listview to display data from details tab:

 public class list extends Activity {

//details details=new details();
 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.list_tab);

    ListView lv=(ListView) findViewById(R.id.listView1);
    //details details=new details();
    //ArrayList mylist=details.getInstance().detaillist;
    MyAdapter adapter=new MyAdapter(this,android.R.layout.simple_list_item_1 ,
            R.id.textView1,
            details.getInstance().detaillist);

    lv.setAdapter(adapter);

//  ArrayAdapter<details> adapter=new ArrayAdapter<details>(this,R.layout.my_list,R.id.textView1,R.id.textView2,));
}
 private class MyAdapter extends ArrayAdapter<String>{

        public MyAdapter(Context context, int resource, int textViewResourceId,
                ArrayList<detail> detaillist) {
            super(context, resource, textViewResourceId);
            // TODO Auto-generated constructor stub
        }
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {

            LayoutInflater inflater=(LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            View row=inflater.inflate(R.layout.my_list, parent,false);
            ArrayList<detail> items=details.getInstance().detaillist;

            ImageView ivR=(ImageView) row.findViewById(R.id.imageView1);
            TextView tvN=(TextView) row.findViewById(R.id.textView1);
            TextView tvA=(TextView) row.findViewById(R.id.textView2);

            tvN.setText(items.get(position).tvName.getText());
            tvA.setText(items.get(position).tvAdr.getText());

            if(items.get(position) != null)
            {
                if(items.get(position).radioValue==0)
                {
                    ivR.setImageResource(R.drawable.t);
                }
                else if(items.get(position).radioValue==1)
                {
                    ivR.setImageResource(R.drawable.t);
                }
                else if(items.get(position).radioValue==2)
                {
                    ivR.setImageResource(R.drawable.t);
                }
            }



            return row;
        }
    }
}

I receive a NullPointException error onmakeDetailList(d) method . where is the problem? I really appreciate your help best regards