Get 500 his page isn’t working when run migrate codeigniter2

Question

I get a 500 page isn’t working when run migrate codeigniter2. I am new here and it's take me days from this bug

Migate controller

<?php

class Migrate extends CI_Controller
{

    public function index()
    {
            $this->load->library('migration');

            if ($this->migration->current() === FALSE)
            {
                    show_error($this->migration->error_string());
            }
    }
}

Config database

<?php  if ( ! defined('BASEPATH')) exit('No direct script access allowed');


$active_group = 'default';
$active_record = TRUE;

$db['default']['hostname'] = 'localhost';
$db['default']['username'] = 'root';
$db['default']['password'] = 'passw';
$db['default']['database'] = 'first_app';
$db['default']['dbdriver'] = 'mysql';
$db['default']['dbprefix'] = '';
$db['default']['pconnect'] = FALSE;
$db['default']['db_debug'] = FALSE;
$db['default']['cache_on'] = FALSE;
$db['default']['cachedir'] = '';
$db['default']['char_set'] = 'utf8';
$db['default']['dbcollat'] = 'utf8_general_ci';
$db['default']['swap_pre'] = '';
$db['default']['autoinit'] = TRUE;
$db['default']['stricton'] = FALSE;

Migration file

<?php

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

class Migration_Add_blog extends CI_Migration {

    public function up()
    {
            $this->dbforge->add_field(array(
                    'blog_id' => array(
                            'type' => 'INT',
                            'constraint' => 5,
                            'unsigned' => TRUE,
                            'auto_increment' => TRUE
                    ),
                    'blog_title' => array(
                            'type' => 'VARCHAR',
                            'constraint' => '100',
                    ),
                    'blog_description' => array(
                            'type' => 'TEXT',
                            'null' => TRUE,
                    ),
            ));
            $this->dbforge->add_key('blog_id', TRUE);
            $this->dbforge->create_table('blog');
    }

    public function down()
    {
            $this->dbforge->drop_table('blog');
    }
}

and when I tried to find buss using echo, the following line cannot run:

$this->load->library('migration'); 

I don't know why?

Please help me .