is_plugin_active() not defined on active plugin, in the thumbnails.php file

You can try this, it works for me 🙂

// We need plugin.php!
        require_once( ABSPATH . 'wp-admin/includes/plugin.php' );

        $plugins = get_plugins();

        foreach ($plugins as $key => $value) {
            if ( $value['Name'] == 'Aest Plugins' ) { // <-- name plugin validate
                if( is_plugin_active( $key ) ){
                    echo "plugin {$value['Name']} is active";   
                }else{
                    echo "plugin {$value['Name']} is desactive";    
                }
            }
        }