致命错误:在非对象上调用成员函数error(）

我正试图遵循phpacademy的OOP PHP教程，但我被卡住了。我已经检查了无数次代码，但我似乎无法弄清楚。其他人也问过类似的问题，但我没有找到适合我的解决方案

我得到错误：

致命错误：调用第5行/var/www/vhosts/nightra.net/httpdocs/index.php中非对象的成员函数error（）

我对OOP比较陌生，所以我相信它是显而易见的。请你们中的任何一位专家为我指明正确的方向好吗？

这是我的代码：

index.php

    <?php
require_once 'core/init.php'; 
$user = DB::getInstance()->get('users', array('username', '=', 'alex'));
if($user->error()){
    echo 'No user';
}else{
    echo 'OK'; 
}

init.php

<?php
session_start();
$GLOBALS['config'] = array(
        'mysql'=>array(
            'host' => '127.0.0.1',
            'username' => 'myusernamehere',
            'password' => 'mypasswordhere',
            'db' => 'mydatabasename'
        ),
        'remember'=>array(
            'cookie_name' =>'hash',
            'cookie_expiry' => 604800
        ),
        'session'=>array(
            'session_name' => 'user'
        )
);
spl_autoload_register(function($class) {
    require_once 'classes/' . $class . '.php';
});
require_once 'functions/sanitize.php';

DB.php

<?php 
class DB {
    private static $_instance = null;
    private $_pdo,
            $_query,
            $_error = false,
            $_results,
            $_count = 0;
    private function __construct() {
        try{
            $this->_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;
    }
 }

Config.php

<?php
class Config {
    public static function get($path = null){
        if($path){
            $config = $GLOBALS['config'];
            $path = explode('/', $path);
            foreach($path as $bit){
                if(isset($config[$bit])){
                    $config = $config[$bit];
                }
            }
            return $config;
        }
        return false;
    }
}