Error in CMS page in Code Igniter

I'm having an error with a CMS developed by someone else in CodeIgniter, the error is as follows

Fatal error: Call to undefined function get_instance() in /home/msinai/public_html/application/errors/error_404.php on line 1

Detail: This page is a CMS administration page, so it should only be shown when logged in, however, anyone can see the error by accessing:

link

I am VERY lazy in backend programming, and I tried to make some modifications to some files, which apparently were successful, I did not move in this 404 error file and this error is there. Follow the code of the page error_404.php

<?php $ci =& get_instance(); ?>
<?php $ci->fixed_content->header(); ?>
<?php $settings = $ci->fixed_content->settings; ?>
<section class="pages contact" style="background-image: url(<?php echo upload_url($settings['banner']); ?>);">
<div class="container">
    <div class="row">
        <h1>Página não encontrada</h1>
        <div class="wrap">

        </div>
    </div>
</div>
</section>
<?php $ci->fixed_content->footer(); ?>

Controller page code for this page Changes: link

PHP Version: 5.3.29

CodeIgniter Version: 1

Just do not know where this error came from or how to solve it, thank you for helping me. Thank you.