WordPress Nemus Slider : PHP Fatal error: Call to a member function add_cap() on a non-object in /PATH/TO/SITE/FOLDER/wp-content/plugins/nemus-slider/nemus-slider.php on line 97

nemus-slider.php

<?php
/*
 * Nemus Slider
 * https://wordpress.org/plugins/nemus-slider/
 * nemus-slider.php line 75
 * Reported issue : https://wordpress.org/support/topic/php-fatal-error-call-to-a-member-function-add_cap-on-a-non-object/
 */

//Capabilities
function nemus_slider_add_caps_to_admin() {
    $caps = array(
        'read',
        'read_nemus-slider',
        'read_private_nemus-sliders',
        'edit_nemus-sliders',
        'edit_private_nemus-sliders',
        'edit_published_nemus-sliders',
        'edit_others_nemus-sliders',
        'publish_nemus-sliders',
        'delete_nemus-sliders',
        'delete_private_nemus-sliders',
        'delete_published_nemus-sliders',
        'delete_others_nemus-sliders',
    );
    $roles = array(
        get_role( 'administrator' ),
        get_role( 'editor' ),
    );
    foreach ($roles as $role) {
        // If there is no editor role you've got a :
        // PHP Fatal error:  Call to a member function add_cap() on a non-object in /PATH/TO/SITE/FOLDER/wp-content/plugins/nemus-slider/nemus-slider.php on line 97
        // So you've got to check before triying to add_cap to an unexisting role :
        if ( $role ) {
            foreach ($caps as $cap) {
                $role->add_cap( $cap );
            }
        }
    }
}
add_action( 'after_setup_theme', 'nemus_slider_add_caps_to_admin' );