问CI get_instance返回null

EN

defined('BASEPATH') OR exit('No direct script access allowed');

class RajaOngkir extends CI_Controller {

private $api_key, $base_url;

public function __construct() {
    // Pastikan bahwa PHP mendukung cURL
    if (!$this->is_curl_exists()) {
        log_message('error', 'cURL Class - PHP was not built with cURL enabled. Rebuild PHP with --with-curl to use cURL.');
    }
    $this->_ci=&get_instance();
    var_dump($this->_ci);
    $this->_ci->load->config('rajaongkir', TRUE);
    // Pastikan Anda sudah memasukkan API Key di application/config/rajaongkir.php
    if ($this->_ci->config->item('api_key', 'rajaongkir') == "") {
        log_message("error", "Harap masukkan API KEY Anda di config.");
    } else {
        $this->api_key = $this->_ci->config->item('api_key', 'rajaongkir');
        $this->base_url = $this->_ci->config->item('api_base_url', 'rajaongkir');
    }
}

Severity: Notice

   Message: Trying to get property of non-object

Filename: controllers/rajaongkir.php

Line Number: 22

Fatal error: Call to a member function config() on a non-object in C:\xampp\htdocs\Koen_CI\application\controllers\rajaongkir.php on line 22