Erreur "NullPointerException"

Salut Tout Le Monde,
je suis débutante en android et j'ai rencontré une exception:NullPointer exception. je n'ai pas arrivé à comprendre la cause du problème. j'ai besoin de votre aide.
voici mon code: (j'ai mis un commentaire au dessus de la ligne qui contient l'erreur)
LoginActivity.java

Code:

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package com.example.myschoolapplicationproject;
 
import java.io.InputStream;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;
 
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;
import org.json.JSONException;
import org.json.JSONObject;
 
import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.os.StrictMode;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
 
//import com.example.androidhive.library.DatabaseHandler;
//import com.example.myschoolapplicationproject.library.UserFunctions;
import com.example.myschoolapplicationproject.*; 
@SuppressLint("NewApi")
public class LoginActivity extends Activity implements OnClickListener{
 
 EditText loginEmail, loginPassword;
 Button btnLogin;
 
 String username,password;
 HttpClient httpclient;
 HttpPost httppost;
 ArrayList<NameValuePair> nameValuePairs;
 HttpResponse response;
 HttpEntity entity;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.login);
 
        if (android.os.Build.VERSION.SDK_INT > 9) {
            StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
            StrictMode.setThreadPolicy(policy);
        }
 
       initialise();
 
    }
       private void initialise(){
       loginEmail=(EditText)findViewById(R.id.loginEmail);
       loginPassword=(EditText)findViewById(R.id.loginPassword);
       btnLogin =(Button) findViewById(R.id.btnLogin);
       btnLogin.setOnClickListener(this);
       }
        // Link to Register Screen
       /* btnLinkToRegister.setOnClickListener(new View.OnClickListener() {
 
            public void onClick(View view) {
                Intent i = new Intent(getApplicationContext(),
                        RegisterActivity.class);
                startActivity(i);
                finish();
            }
        });*/
	@Override
	public void onClick(View v) {
		// TODO Auto-generated method stub
		httpclient= new DefaultHttpClient();
		httppost= new HttpPost ("http://10.0.2.2/AndroidConnect/get_user.php");
	    username=loginEmail.getText().toString();
	    password=loginPassword.getText().toString();
 
 
	    try{
 
            nameValuePairs= new ArrayList<NameValuePair>();
 
		    nameValuePairs.add(new BasicNameValuePair("username",username));
		    nameValuePairs.add(new BasicNameValuePair("password",password));
 
	    	httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
	    	response=httpclient.execute(httppost);
	    	if(response.getStatusLine().getStatusCode()==200){
	    		entity= response.getEntity();
	    		if(entity!=null){
	    			InputStream instream= entity.getContent();
	    			//la ligne suivante est la source de l'erreur
	    			JSONObject jsonResponse = new JSONObject(convertStreamToString(instream));
		   		     Log.e("connnnnn errooooor","connnnn erroooooor");
	    			String retUser = jsonResponse.getString("user");
	    			String retPass=jsonResponse.getString("pass");
	    			if(username.equals(retUser)&&password.equals(retPass)){
 
	    				SharedPreferences sp=getSharedPreferences("logindetails",0);
	    				SharedPreferences.Editor spedit=sp.edit();
	    				spedit.putString("user",username);
	    				spedit.putString("password",password);
	    				spedit.commit();
	    				Toast.makeText(getBaseContext(), "SUCCES§", Toast.LENGTH_SHORT);
	    				Log.d("success","welcome");
	    			}else{
	    				Log.d("error","invalid log details");
	    				Toast.makeText(getBaseContext(), "Invalid Login Details", Toast.LENGTH_SHORT).show();
	    			}
	    		}
	    	}
	    }catch(Exception e){
	     e.printStackTrace();	
	     Toast.makeText(getBaseContext(), "Connection Error",Toast.LENGTH_SHORT).show();
	    }
	}
		private Map convertStreamToString(InputStream instream) {
			// TODO Auto-generated method stub
			return null;
		}
    }

voici mon code php :get_user.php

Code:

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<?php
error_reporting(E_ALL ^ E_NOTICE ^ E_WARNING);
 
$dbhost="localhost";
$dbuser="root";
$dbpass="pfe";
$dbdb="gestionecole";
 
$connect=mysql_connect($dbhost,$dbuser,$dbpass) or die("connection error");
 
mysql_select_db($dbdb) or die("database selection error");
$Login = $_POST['Login'];
$MotDePasse = $_POST['MotDePasse'];
 
$query = mysql_query("SELECT * FROM compte_utilisateur WHERE  Login = $Login AND MotDePasse = $MotDePasse");
$num = mysql_num_rows($query);
 
if($num == 1){
while($list-mysql_fetch_assoc($query)){
$output = $list;
echo json_encode($output);
}
mysql_close();
}
?>

j'attends vos aides et merci d'avance