Which is the correct way to conditionally enqueue a CSS file?

Question

get_queried_object returns NULL inside a function hooked to wp_enqueue_scripts action hook when going to an nonexistent category URL on my website. If the category exists, the error is not shown and I think it does not exist.

I need it to conditionally load a CSS file for better modularization, not in the admin area but for the end-user.

What is the correct way to do this?

The error shown in the HTML:

Notice: Trying to get property 'term_id' of non-object in /var/www/html/wp-content/themes/custom-theme/functions.php on line 193

The code starting at line 193:

if (get_queried_object()->term_id === 3 ||
        (count(get_the_category()) > 0 && get_the_category()[0]->slug == 'arta')) {
    wp_enqueue_style( 'twentytwenty-style-2', get_stylesheet_directory_uri() . '/style-arta.css', array(), $theme_version );
}

Answer 1

I agreed with @PatJ — you should check if get_queried_object() returns an object or not.

But you could simplify your code by simply using is_category() and in_category() which are conditional tags in WordPress:

if ( is_category( 3 ) || in_category( 'arta' ) ) {
    wp_enqueue_style( 'twentytwenty-style-2', get_stylesheet_directory_uri() . '/style-arta.css', array(), $theme_version );
}

Answer 2

According to the documentation, get_queried_object() is a wrapper for WP_Query::get_queried_object(), which will return null if there's no such object.

A simple way to check that you've got a non-null return is using empty():

$object = get_queried_object();
if ( 
    ! empty( $object) &&
    ( 
        $object->term_id === 3 ||
        (
            count( get_the_category() ) > 0 &&
            get_the_category()[0]->slug == 'arta'
        )
    ) 
) {
    wp_enqueue_style( 'twentytwenty-style-2', get_stylesheet_directory_uri() . '/style-arta.css', array(), $theme_version );
}