I have writen a code and there is a error i get could anyone tell me whats wrong please?

error Fatal error: Call to a member function error() on a non-object in C:\xampp\htdocs\loginsistem\index.php on line 6

but it should say OK! if i m right

init.php:

 array(
        'host' => '127.0.0.1',
        'username' => 'root',
        'password' => '',
        'db' => 'lr'
    ),
    'remember' => array(
        'cookie_name' => 'hash',
        'cookie_expiry' => 604800
    ),
    'session' => array(
        'session_name' => 'user'
    )
);

spl_autoload_register(function($class) {
    require_once 'classes/' . $class . '.php';

});

require_once 'functions/sanitize.php';
?>

index.php:

get('users', array('username', '=', 'grega'));

if($user->error()) {
    echo 'No user';
} else {
    echo 'OK!';
}

?>

DB.php

_pdo = new PDO('mysql:host=' . Config::get('mysql/host') .';dbname=' . Config::get('mysql/db'), Config::get('mysql/username'), Config::get('mysql/password'));
        } catch (PDOException $e) {
        die($e->getMessage());
        }
    }

    public static function getInstance() {
        if(!isset(self::$_instance)) {
            self::$_instance = new DB();
        }
        return self::$_instance;
    }

    public function query($sql, $params = array()) {
        $this->_error = false;
        if($this->_query = $this->_pdo->prepare($sql)) {
            $x = 1;
            if(count($params)) {
                foreach($params as$param) {
                    $this->_query->bindValue($x, $param);
                    $x++;
                }
            }
            if($this->_query->execute()) {
                $this->_results = $this->_query->fetchAll(PDO::FETCH_OBJ);
                $this->_count = $this->_query->rowCount();
            } else {
                $this->_error = true;
            }
        }
        return $this;
    }

    public function action($action, $table, $where = array()) {
        if(count($where) === 3) {
            $operators = array('=', '>', '<', '>=', '<=');

            $field     = '$where[0]';
            $operator  = '$where[1]';
            $value     = '$where[2]';

            if(in_array($operator, $operators)) {
                $sql = "{$action} FROM {$table} WHERE {$field} {$operator} ?";

                if($this->query($sql, array($value))->error()) {
                    return $this;
                }
            }
        }
        return false;
    }
    public function get($table, $where) {
        return $this->action('SELECT *', $table, $where);
    }
    public function delete($table, $where) {
        return $this->action('DELETE', $table, $where);
    }

    public function error() {
        return $this->_error;
    }

}