Problem with creating a user in the application?

Question

Android

The people who know what the problem is, everything seems to be done correctly, the application just crashes when the button is pressed exactly when the fields are filled in, help the newbie, otherwise I already broke my head and could not understand what the problem could be)

package com.example.user.testv2;

import android.content.Intent;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;

public class RegistrationActivity extends AppCompatActivity implements View.OnClickListener{

    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener mAuthListener;

    private EditText ETemail;
    private EditText ETpassword;

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

        ETemail = (EditText) findViewById(R.id.et_email_reg);
        ETpassword = (EditText) findViewById(R.id.et_password_reg);

        findViewById(R.id.registration_reg).setOnClickListener(this);
        findViewById(R.id.go_authent_reg).setOnClickListener(this);
    }

    public void reg(String email , String password)
    {
        mAuth.createUserWithEmailAndPassword(email, password).addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if (task.isSuccessful()) {
                    Toast.makeText(RegistrationActivity.this, "Регистрация успешна", Toast.LENGTH_SHORT).show();
                } else
                    Toast.makeText(RegistrationActivity.this, "Регистрация провалена", Toast.LENGTH_SHORT).show();
            }
        });
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.registration_reg:
                if (ETemail.getText().length() != 0 && ETpassword.getText().length() != 0) {

                    reg(ETemail.getText().toString(), ETpassword.getText().toString());

                } else {
                    Toast.makeText(RegistrationActivity.this, "Заполните все поля", Toast.LENGTH_SHORT).show();
                }
                break;
            case R.id.go_authent_reg:
                Intent intent = new Intent(this, MainActivity.class);
                startActivity(intent);
                break;
        }
    }
}