Creating Walker class to style main menu bar gives unknown error

I’m trying to create a Walker class to style my top main menu bar. I’m following this guide and code sample:
https://www.youtube.com/watch?v=ArEmwJV6M7s
https://github.com/Alecaddd/WordPress101/blob/master/lesson_14/inc/walker.php

When I implement it on my website using below code snippets, I consitently get the following error:

English translation from Dutch:
“A critical error has occurred on this site. Check your site administrator email inbox for instructions. Learn more about troubleshooting issues in WordPress.”

Code

functions.php

require get_template_directory() . '/inc/walker.php';

walker.php

<?php
/* Collection of Walker classes */

class MeterInsight_Menu_Walker extends Walker_Nav_Menu {
    function start_lvl(&$output, $depth=0, $args=null) { //ul
        $indent = str_repeat("\t",$depth);
        $submenu = ($depth > 0) ? ' sub-menu' : '';
        $output .= "\n$indent<ul class=\"dropdown-menu$submenu depth_$depth\">\n";
    }

    function start_el(&$output, $item, $depth=0, $args=[], $id=0) {
    }

    function end_el() {
    }

    function end_lvl() {
    }
}
?>

menu.php:

<?php
wp_nav_menu(
    array(
        'theme_location' => 'primary',
        'menu_class' => 'navbar-nav ms-auto',
        'container' => 'nav',
        'container_class' => 'collapse navbar-collapse',
        'menu' => 'primary',
        'walker' => new MeterInsight_Menu_Walker()
    )
);
?>

How can I fix this? Or where can I get a more elaborate error message?

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