I did say it was untested, right? 
I canāt offer you too much help with debugging right now, but hereās an updated version that should fix that issue as well as removing a redundant check in the second action:
add_action( 'create_category', 'discourse_create_category', 10, 1 );
function discourse_create_category( $term_id ) {
$category = get_term_by( 'id', $term_id, 'category' );
$name = $category->name;
$path = '/categories.json';
// Use the WPDiscourse plugin to generate a new category
$utils = new \WPDiscourse\Utilities\PublicPluginUtilities();
$utils->discourse_request(
$path,
[
'method' => 'POST',
'name' => $name,
]
);
}
add_action( 'set_object_terms', 'discourse_set_object_terms', 10, 1 );
function discourse_set_object_terms( $object_id ) {
if ( ! metadata_exists( 'post', $object_id, 'publish_to_discourse' ) ) {
// Enable publishing in Discourse, during initial save only.
update_post_meta( $object_id, 'publish_to_discourse', true );
$discourse_publish_option = get_option( 'discourse_publish' );
// set the Discourse category based on WP category
if ( is_array( $discourse_publish_option ) ) {
$categories_list = wp_get_post_categories( $object_id );
update_post_meta( $object_id, 'publish_post_category', $categories_list[0]->name );
}
}
}
7.4 is only about three months away from EOL, so you should really start planning to upgrade your hosting environment, if you havenāt already 