WP-AppKit - Custom template if post has a specific category

add-custom-data.php

<?php
/*
 * @desc Add custom data to what is returned by the web services. All custom data will be available to the JS API.
 * @param $post_data
 * @param $post
 * @param $component
 */
function wpak_add_custom_data( $post_data, $post, $component ) {
    
    $taxonomy = 'category'; // We search for categories
    $terms = get_the_terms( $post->ID, $taxonomy ); // Get the post categories
    $cat_list = wp_list_pluck( $terms, 'slug' ); // Get an array of slugs only

    // Add category list to post data returned by the web service
    $post_data['categories'] = $cat_list;
    
    return $post_data; // Return the modified $post_data

}

// Hook into post content web service feedback
add_filter( 'wpak_post_data', 'wpak_add_custom_data', 10, 3 );

?>

functions.js

App.filter( 'template', function( template, current_screen ) {

    // Detect single
    if ( current_screen.screen_type === 'single' ) {

        // Get post categories (added in add-custom-data.php)
        var post_categories = current_screen.data.post.categories;

        // Call template for specific category's slug
        if ( post_categories.indexOf('any-cat-slug-here') !== -1 ){
            template = 'custom-single-template'; // corresponds to custom-single-template.html
        }

    }

    return template;

});

I get this error on console
Uncaught TypeError: Cannot read property 'indexOf' of undefined