My application in android studio is closing, while consuming webservice [closed]

I have a problem in my application, when consuming a web service service, the application is closing.

The idea of my application is that I consume this web service to validate my login, and returning a message approving or not, I do not have Android skills, I'm new to programming it so I do not know if I'm doing something right. I would like you to help me through this stage of the project.

This code refers to MainActivity.java

package com.example.hotsystems.hs_celulas;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import org.json.JSONException;
import org.json.JSONObject;

public class MainActivity extends Activity {

    String asw;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_hs_cell);

        //Chamar os objetos
        final EditText email = (EditText) findViewById(R.id.TXT_EMAIL_LOGIN);
        final EditText senha = (EditText) findViewById(R.id.TXT_SENHA_LOGIN);
        final EditText cod_igrej = (EditText) findViewById(R.id.COD_IGREJ_LOGIN);
        Button entrar = (Button) findViewById(R.id.BTN_ENTRA_APLIC);
        Button sair = (Button) findViewById(R.id.BTN_SAIRX_APLIC);

        entrar.setOnClickListener(new View.OnClickListener(){

            @Override
            public void onClick(View view) {
                Usuario usr = new Usuario();

                String json = generateJson(usr);
                String result = callServer("send-json", json);

                if(result.equals("3")){
                    Log.i("MainActivity", result);
                    //Toast.makeText(MainActivity.this, "Código da igreja inesistente, por favor escreva um código válido." , Toast.LENGTH_SHORT).show();
                }else if (result.equals("2")){
                    Log.i("MainActivity", result);
                    //Toast.makeText(MainActivity.this, "Senha incorreto, por favor escreva uma senha válida." , Toast.LENGTH_SHORT).show();
                }else if (result.equals("1")){
                    Log.i("MainActivity", result);
                    //Toast.makeText(MainActivity.this, "E-mail incorreto, por favor escreva um email válido." , Toast.LENGTH_SHORT).show();
                }else{
                    Intent it = new Intent(MainActivity.this, Home.class);
                    startActivity(it);
                }

            }
        });

        sair.setOnClickListener(new View.OnClickListener(){

            @Override
            public void onClick(View view) {
                finish();
            }
        });
    }

    public String generateJson(Usuario usr){
        final EditText email = (EditText) findViewById(R.id.TXT_EMAIL_LOGIN);
        final EditText senha = (EditText) findViewById(R.id.TXT_SENHA_LOGIN);
        final EditText cod_igrej = (EditText) findViewById(R.id.COD_IGREJ_LOGIN);

        JSONObject jo = new JSONObject();

        String strEmail = email.getText().toString();
        String strSenha = senha.getText().toString();
        String strCod = cod_igrej.getText().toString();

        try{
            jo.put("email", strEmail);
            jo.put("senha", strSenha);
            jo.put("cod_igrej", strCod);
        }catch (JSONException e){
            e.printStackTrace();
        }

        return(jo.toString());
    }

    public Usuario degenerateJson(String data){
        Usuario usr = new Usuario();

        try{
            JSONObject jo = new JSONObject(data);

            usr.setEmail(jo.getString("email"));
            usr.setEmail(jo.getString("senha"));
            usr.setEmail(jo.getString("cod_igrej"));

            /*jo.put("email", Usuario.getEmail());
            jo.put("senha", Usuario.getSenha() );
            jo.put("cod_igrej", Usuario.getCod());*/


        }catch (JSONException e){
            e.printStackTrace();
        }
        return(usr);
    }

    private String callServer(final String method, final String data){

        new Thread(){

            public void run(){
                asw = Connection.getSetDataWeb("http://192.168.1.20/renan/process.php", method, data);

                if (data.isEmpty()){
                    degenerateJson(asw);
                }
            }
        }.start();
        return asw;
    }

}

I would also like to be told if the method as I call the next screen after login is implemented in the best way. Below is my web server service.

<?php
require_once('Usuario.php');
include('conexao.php');


if(strcmp('send-json', $_POST['method']) == 0)
{ // SEND

    $Usuario = utf8_encode($_POST['json']);
    $Usuario = json_decode($Usuario);

    $email = $Usuario->email;
    $senha = $Usuario->senha;
    $cod_igrej = $Usuario->cod_igrej;



    $sql = "SELECT COD_IDENT_IGREJ, TXT_NOMEX_IGREJ, FLG_STATU_IGREJ FROM tbl_IGREJAS WHERE COD_IDENT_IGREJ = '".$cod_igrej."'";
    $sql2 = "SELECT COD_IDENT_USUAR, TXT_EMAIL_LOGIN, TXT_SENHA_LOGIN FROM tbl_USUARIOS WHERE TXT_EMAIL_LOGIN = '".$email."'";

    $resultado = mysql_query($sql);

    if (mysql_num_rows($resultado) == 0) 
    {
            $val= '003';
    }else
    {
            $resultado = mysql_query($sql2);
            if (mysql_num_rows($resultado) == 0 ) 
            {
                $val='001';
            }else 
            {
                $usuario = mysql_fetch_object($resultado);
                if ($senha == $usuario->TXT_SENHA_LOGIN) {
                    $val='000';
                }else{
                    $val= '002';
                }
            }
    }





    switch ($val) 
    {
        case '001':
        /*E-mail incorreto, por favor escreva um email válido.*/
            echo '1';
            break;

        case '002':
        /*Senha incorreta, por favor escreva uma senha válido.*/
            echo '2';
            break;

        case '003':
        /*Codigo da igreja inesistente, por favor escreva um codigo válido.*/
            echo '3';
            break;


        default:
        /*Login efetuado com sucesso*/
            echo '0';
            break;
    };

}


else if(strcmp('get-json', $_POST['method']) == 0)
{ // GET
    $Usuario = new Usuario();
    $Usuario->setEmail(utf8_encode('Renan'));
    $Usuario->setSenha('12345');
    $Usuario->setCod('001');


    echo json_encode($Usuario->getDataJSON());
}

? >

The basic error here is the

switch (result){
                case "3":
                    Toast.makeText(MainActivity.this, "Código da igreja inesistente, por favor escreva um código válido." , Toast.LENGTH_SHORT).show();
                    break;
                case "2":
                    Toast.makeText(MainActivity.this, "Senha incorreto, por favor escreva uma senha válida." , Toast.LENGTH_SHORT).show();
                    break;
                case "1":
                    Toast.makeText(MainActivity.this, "E-mail incorreto, por favor escreva um email válido." , Toast.LENGTH_SHORT).show();
                    break;
                default:
                    Intent it = new Intent(MainActivity.this, Home.class);
                    startActivity(it);
            }

I need to compare with the response coming from the webservice, but then that is giving the flaw. Logcat is:

 java.lang.NullPointerException
        at com.example.hotsystems.hs_celulas.MainActivity$1.onClick(MainActivity.java:40)
        at android.view.View.performClick(View.java:4438)
        at android.view.View$PerformClick.run(View.java:18422)
        at android.os.Handler.handleCallback(Handler.java:733)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5017)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
        at dalvik.system.NativeStart.main(Native Method)