Fatal error: Call a member function prepare () on null in

I have the following classes in PHP Database.class.php

<?php  
abstract class Database {

    private $dataBaseHandler;
    private $error;
    private $statement;

    protected function __construct() {
        $options = array(
        PDO::ATTR_PERSISTENT => true,
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION
        );
        try {
            $this->dataBaseHandler = new PDO('mysql:host='. DB_HOST.';dbname='. DB_NAME, DB_USER, DB_PASS, $options);
            $this->dataBaseHandler->exec('SET NAMES UTF8');
        }
        catch(PDOException $e) {
            $this->error = $e->getMessage();
        }
    }

    protected function query($query) {
        $this->statement = $this->dataBaseHandler->prepare($query);
    }

    protected function bind($param, $value, $type = null) {
        if (is_null($type)) {
            switch (true) {
                case is_int($value):
                    $type = PDO::PARAM_INT;
                break;
                case is_bool($value):
                    $type = PDO::PARAM_BOOL;
                break;
                case is_null($value):
                    $type = PDO::PARAM_NULL;
                break;
                default:
                    $type = PDO::PARAM_STR;
            }
        }
        $this->statement->bindValue($param, $value, $type);
    }

    protected function execute() {
        return $this->statement->execute();
    }

    protected function resultset() {
        $this->execute();
        return $this->statement->fetchAll(PDO::FETCH_ASSOC);
    }

    protected function single() {
        $this->execute();
        return $this->statement->fetch(PDO::FETCH_ASSOC);
    }

    protected function rowCount() {
        return $this->statement->rowCount();
    }

    protected function lastInsertId() {
        return $this->dataBaseHandler->lastInsertId();
    }

    protected function beginTransaction() {
        return $this->dataBaseHandler->beginTransaction();
    }

    protected function endTransaction() {
        return $this->dataBaseHandler->commit();
    }

    protected function cancelTransaction() {
        return $this->dataBaseHandler->rollBack();
    }

    protected function debugDumpParams() {
        return $this->statement->debugDumpParams();
    }

    protected function closeCursor() {
        $this->statement->closeCursor();
    }
}?>

and the ControllerRegister.class

<?php
include_once 'classes/Database.class.php';

class ControllerRegister extends Database {

    private $firstName, $lastName, $email, $password, $confPassword;

    public function __construct() {
        $this->firstName = filter_input(INPUT_POST,'firstname',FILTER_SANITIZE_STRING);
        $this->lastName = filter_input(INPUT_POST,'lastname',FILTER_SANITIZE_STRING);
        $this->email = filter_input(INPUT_POST,'email',FILTER_SANITIZE_STRING);
        $this->password = filter_input(INPUT_POST,'password',FILTER_SANITIZE_STRING);
        $this->confPassword = filter_input(INPUT_POST,'confPassword',FILTER_SANITIZE_STRING);
        $this->checkPassword();
    }

    private function checkPassword() {
        if($this->password == $this->confPassword)
        {
            $this->registerPassword();
        }else{
            echo 'Different password and password confirmation.';
            return false;
        }
    }

    private function hashPassword() {
        $this->password = password_hash($this->password,PASSWORD_DEFAULT);
    }

    private function registerPassword() {
        // Define configuration
        define('DB_HOST', 'localhost');
        define('DB_USER', 'root');
        define('DB_PASS', '');
        define('DB_NAME', 'exemple');

        $this->hashPassword();
        $this->query('INSERT INTO users VALUES (NULL, :first_name, :last_name, :email, :password)');
        $this->bind(':password', $this->password);
        $this->bind(':firstName',$this->firstName);
        $this->bind(':lastName', $this->lastName);
        $this->bind(':email', $this->email);
        $this->bind(':password',$this->password);
        $this->execute();
    }
}
$new = new ControllerRegister();?>

The data is passed to ControllerRegister through an html form and when I submit the data I get the error Fatal error: Call to a member function prepare() on null in C:\Program Files (x86)\EasyPHP-Devserver-17\eds-www\web\settings\classes\Database.class.php on line 23 . I'm a whole morning breaking my head, please help me, Grate!