problem with firebase authentication

apply plugin: 'com.android.application'

android {
    compileSdkVersion 27
    defaultConfig {
        applicationId "com.justforum.yotamarker.fb3"
        minSdkVersion 18
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:27.1.1'
    implementation 'com.android.support.constraint:constraint-layout:1.1.2'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
    implementation 'com.google.firebase:firebase-core:16.0.0'
    implementation 'com.google.firebase:firebase-auth:9.2.1'

}
apply plugin: 'com.google.gms.google-services'

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <LinearLayout
        android:layout_centerVertical="true"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">

        <EditText
            android:id="@+id/editTextEmail"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_margin="15dp"
            android:hint="Enter email"
            android:inputType="textEmailAddress" />

        <EditText
            android:id="@+id/editTextPassword"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_margin="15dp"
            android:hint="Enter password"
            android:inputType="textPassword" />

        <Button
            android:id="@+id/buttonSignup"
            android:onClick="registerClick"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_margin="15dp"
            android:text="Signup" />

    </LinearLayout>

</android.support.constraint.ConstraintLayout>

package com.justforum.yotamarker.fb3;

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

import com.google.firebase.auth.FirebaseAuth;

public class MainActivity extends AppCompatActivity {
    private EditText editTextEmail;
    private EditText editTextPassword;
    private Button buttonSignup;
    private ProgressDialog progressDialog;


    //defining firebaseauth object
    private FirebaseAuth firebaseAuth;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        firebaseAuth = FirebaseAuth.getInstance();

        //initializing views
        editTextEmail = (EditText) findViewById(R.id.editTextEmail);
        editTextPassword = (EditText) findViewById(R.id.editTextPassword);
        progressDialog = new ProgressDialog(this);
    }
    private void registerUser(){

        //getting email and password from edit texts
        String email = editTextEmail.getText().toString().trim();
        String password  = editTextPassword.getText().toString().trim();

        //checking if email and passwords are empty
        if(TextUtils.isEmpty(email)){
            Toast.makeText(this,"Please enter email",Toast.LENGTH_LONG).show();
            return;
        }

        if(TextUtils.isEmpty(password)){
            Toast.makeText(this,"Please enter password",Toast.LENGTH_LONG).show();
            return;
        }

        //if the email and password are not empty
        //displaying a progress dialog

        progressDialog.setMessage("Registering Please Wait...");
        progressDialog.show();

        //creating a new user
        firebaseAuth.createUserWithEmailAndPassword(email, password)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        //checking if success
                        if(task.isSuccessful()){
                            //display some message here
                            Toast.makeText(MainActivity.this,"Successfully registered",Toast.LENGTH_LONG).show();
                        }else{
                            //display some message here
                            Toast.makeText(MainActivity.this,"Registration Error",Toast.LENGTH_LONG).show();
                        }
                        progressDialog.dismiss();
                    }
                });
    }
    public void registerClick(View view) {
        registerUser();
    }
}