Error in showing alert dialog while clicking on a list view item

Error in showing alert dialog while clicking on a list view item.

The code is given below.. this is a program done in android sqlite.The data retrieved is shown in the list view but the alert dialogue does not working properly for edit and delete.

please help me with the code......

import java.util.ArrayList;
import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.PopupWindow;

public class Display11 extends Activity {
    ListView listview;
    PopupWindow popup;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.display1);

        listview=(ListView)findViewById(R.id.listView1);
        Intent in=getIntent();
        ArrayList<String> a=in.getStringArrayListExtra("Key");
        ArrayAdapter<String> dis=new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,a);
        listview.setAdapter(dis);
        listview.setTextFilterEnabled(true);
        listview.setOnItemClickListener(new AdapterView.OnItemClickListener() {

            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position,long rowId) {
                // TODO Auto-generated method stub

        ///public void OnItemClick(AdapterView<?> parent, View view, int position,long rowId){
                int itemposition = position;
                String itemValue = (String)listview.getItemAtPosition(position); 
                // TODO Auto-generated method stub



                AlertDialog.Builder alert  = new AlertDialog.Builder(getApplicationContext());
                /*LayoutInflater inflater = LayoutInflater.from(getApplicationContext());
                View view1 =inflater.inflate(R.layout.alert_dialog,null);
                alert.setView(view1);*/
                //ad.setView(inflater.inflate(R.layout.alert_dialog, null));
                popup = new PopupWindow(getBaseContext());
                //popup.setContentView(alert_)
                alert.setMessage("Database Manager");

                alert.setIcon(R.drawable.ic_launcher);
                alert.setTitle("Terms of Service");

                alert.setPositiveButton("edit",new DialogInterface.OnClickListener(){

                    public void onClick(DialogInterface arg0, int arg1) {
                        // TODO Auto-generated method stub
                        Intent positiveActivity=new Intent(getApplicationContext(),com.cerebtec.contactdb.PositiveActivity.class);
                    startActivity(positiveActivity);
                    }});

                alert.setNegativeButton("delete",new DialogInterface.OnClickListener(){

                    public void onClick(DialogInterface dialog, int which) {
                        // TODO Auto-generated method stub
                        Intent negativeActivity=new Intent(getApplicationContext(),com.cerebtec.contactdb.NegativeActivity.class);
                    startActivity(negativeActivity);
                    }});
                //ad.setNeutralButton("Cancel",null);
                //ad.setCancelable(false);


                //AlertDialog alert=ad.create();
                //alert.setView(view1);
                alert.create();
                alert.show();
            }});
        }}