Java.lang.nullPointerException: попытка вызвать виртуальный метод int java.lang.string.hashCode () для ссылки на пустой объект

Мне нужна помощь с этим, пожалуйста.
Я делаю деятельность, где можно зарегистрироваться и войти. При регистрации я следил за каждой процедурой, которой, как мне кажется, я должен был следовать. Но у меня все еще есть ошибка, когда я нажал кнопку регистрации. Я не знаю, откуда возникла ошибка, поэтому я поймал ошибку на registerclick метод, и это дало мне эту ошибку Java.lang.nullPointerException: Attempt to invoke virtual method 'int java.lang.string.hashCode() on a null object reference, Пожалуйста, это не дубликат какого-либо вопроса, так как я просмотрел каждый вопрос, который выглядит так. Все еще нет решения. Это мой класс констант

package tk.trythisout.learnit;public class Constants {
private static final String ROOT_URL = "192.168.33.149/and/req/";
public static final String URL_REGISTER = ROOT_URL+"register.php";
}

RegisterActivity1.java

package tk.trythisout.learnit;

import android.app.ProgressDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.Toast;

import com.android.volley.AuthFailureError;
import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;
import com.android.volley.toolbox.Volley;

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

import java.util.HashMap;
import java.util.Map;
import java.util.Objects;

public class RegisterActivity1 extends AppCompatActivity {
EditText FirstName, LastName, Email, IndexNumber, Password, Repassword, PhoneNumber;
Spinner Level, Program;
Button Register;
//    ProgressDialog progressDialog;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_register1);
FirstName = (EditText) findViewById(R.id.txtFirstName);
LastName = (EditText) findViewById(R.id.txtLastName);
Email = (EditText) findViewById(R.id.txtEmail);
IndexNumber = (EditText) findViewById(R.id.txtIndexNumber);
Password = (EditText) findViewById(R.id.txtPassword);
Repassword = (EditText) findViewById(R.id.txtRePassword);
PhoneNumber = (EditText) findViewById(R.id.txtPhoneNumber);
Level = (Spinner) findViewById(R.id.spinLevel);
Program = (Spinner) findViewById(R.id.spinProgram);
Register = (Button) findViewById(R.id.btnRegister);
//        progressDialog = new ProgressDialog(this);
Register.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
try {
registerClick();
} catch (Exception e) {
Toast.makeText(RegisterActivity1.this, "error: "+e, Toast.LENGTH_LONG).show();
}
}
});

}

private void registerClick() throws Exception  {
final String firstName = FirstName.getText().toString().trim();
final String lastName = LastName.getText().toString().trim();
final String email = Email.getText().toString().trim();
final String indexnumber = IndexNumber.getText().toString().trim();
final String password = Password.getText().toString().trim();
final String repassword = Repassword.getText().toString().trim();
final String phonenumber = PhoneNumber.getText().toString().trim();
final String level = Level.getSelectedItem().toString();
final String program = Program.getSelectedItem().toString();
//        progressDialog.setMessage("Registering User...");
//        progressDialog.show();

StringRequest stringRequest = new StringRequest(Request.Method.POST, Constants.URL_REGISTER, new Response.Listener<String>() {
@Override
public void onResponse(String response) {
//                    progressDialog.dismiss();
try {
JSONObject jsonObject = new JSONObject(response);
Toast.makeText(RegisterActivity1.this, jsonObject.getString("message"), Toast.LENGTH_LONG).show();
} catch (JSONException e) {

}

}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
//                    progressDialog.hide();
Toast.makeText(RegisterActivity1.this, "too bad: "+error, Toast.LENGTH_LONG).show();

}
}) {
@Override
protected Map<String, String> getParams() throws AuthFailureError {
Map<String, String> params = new HashMap<>();
params.put("indexnumber", indexnumber);
params.put("firstName", firstName);
params.put("lastName", lastName);
params.put("level", level);
params.put("email", email);
params.put("program", program);
params.put("password", password);
params.put("phonenumber",phonenumber);
return params;
}
};
RequestQueue requestQueue = Volley.newRequestQueue(this);
requestQueue.add(stringRequest);}
}

register.php

<?php

require_once '../includes/dbOperation.php';

$response = array();

if ($_SERVER['REQUEST_METHOD']=='POST') {
# code...
if (isset($_POST['indexnumber']) and isset($_POST['firstname']) and isset($_POST['lastname'])
and isset($_POST['level']) and isset($_POST['email'])
and isset($_POST['program']) and isset($_POST['password']) and isset($_POST['phonenumber']))
{
# code...
$db = new dbOperation();
if ($db->createUser(
$_POST['indexnumber'], $_POST['firstname'], $_POST['lastname'], $_POST['level'], $_POST['email'], $_POST['program'], $_POST['password'], $_POST['phonenumber'])) {
# code...
$response['error'] = false;
$response['message'] = 'Successfully Registered';

}else{
$response['error'] = true;
$response['message'] = 'Unknown error occurred';
}
}else{
$response['error'] = true;
$response['message'] = 'All fields are required, Pls input correct values into the fields';
}
}
echo json_encode($response);

dboperation.php

<?php

class dbOperation{
private $con;

function __construct(){
require_once dirname(__FILE__).'/DbConnect.php';
$db = new DbConnect();
$this->con = $db->connect();
}

public function createUser($indexnumber, $firstname, $lastname, $level, $email, $program, $password, $phonenumber)
{
# code...
$password = md5($password);
$stmt = $this->con->prepare("INSERT INTO `students`(`sn`, `IndexNumber`, `firstName`, `LastName`, `level`, `email`, `department`, `password`, `phonenumber`) VALUES (NULL,?,?,?,?,?,?,?,?)");
$stmt->bind_param("ssssssss",$indexnumber, $firstname, $lastname, $level, $email, $program, $password, $phonenumber);

if ($stmt->execute()) {
# code...
return true;
}else{
return false;
}
}
}

Пожалуйста помоги. Я застрял!

"main@2" prio=5 waiting
java.lang.Thread.State: WAITING
at com.android.volley.Request.<init>(Request.java:136)
at com.android.volley.toolbox.StringRequest.<init>(StringRequest.java:43)
at tk.trythisout.learnit.RegisterActivity1$4.<init>(RegisterActivity1.java:0)
at tk.trythisout.learnit.RegisterActivity1.registerClick(RegisterActivity1.java:74)
at tk.trythisout.learnit.RegisterActivity1.access$000(RegisterActivity1.java:27)
at tk.trythisout.learnit.RegisterActivity1$1.onClick(RegisterActivity1.java:52)
at android.view.View.performClick(View.java:4756)
at android.view.View$PerformClick.run(View.java:19749)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Method.java:-1)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

это все, что я получил ..

this = {RegisterActivity1$4@4262} "[ ] 192.168.33.149/and/req/register.php 0x0 NORMAL null"this$0 = {RegisterActivity1@4057}
mListener = null
mCacheEntry = null
mErrorListener = {RegisterActivity1$3@4263}
mEventLog = null
mRequestQueue = null
mRetryPolicy = {DefaultRetryPolicy@4271}
mSequence = null
mTag = null
mUrl = "192.168.33.149/and/req/register.php"mRequestBirthTime = 0
mCanceled = false
mDefaultTrafficStatsTag = 0
mMethod = 1
mResponseDelivered = false
mShouldCache = true
shadow$_klass_ = {Class@3966} "class tk.trythisout.learnit.RegisterActivity1$4"shadow$_monitor_ = -1782153838
Exception = {NullPointerException@4261}
cause = {NullPointerException@4261} "java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.hashCode()' on a null object reference"detailMessage = "Attempt to invoke virtual method 'int java.lang.String.hashCode()' on a null object reference"value = {char[93]@4298}
count = 93
hashCode = 154031741
offset = 0
shadow$_klass_ = {Class@1531} "class java.lang.String"shadow$_monitor_ = -1407867750
stackState = {Object[17]@4287}
stackTrace = {StackTraceElement[0]@4288}
suppressedExceptions = {Collections$EmptyList@4289}  size = 0
shadow$_klass_ = {Class@3006} "class java.lang.NullPointerException"shadow$_monitor_ = -2087831779
email = "[email protected]"firstName = "busaf"indexnumber = "afa"lastName = "adf"level = "300"password = "qwerty"phonenumber = "5556321"program = "BSc. Midwifery"x0 = 1
x1 = "192.168.33.149/and/req/register.php"x2 = {RegisterActivity1$2@4273}
x3 = {RegisterActivity1$3@4263}