MySQL triger to set foreign key to null - mysql

Question

I have table named Webpage_Categories (MySQL)
Webpage_Categories
-----------------
webpage_catgegory_id | parent_id | name
where parent_id field is foreign key, which references the webpage_category_id field.
Here is the create table statement:
CREATE TABLE `webpage_category` (
`webpage_category_id` int(11) NOT NULL AUTO_INCREMENT,
`parent_id` int(11) DEFAULT NULL,
`name` varchar(255) NOT NULL,
`name_en` varchar(255) DEFAULT NULL,
`thumbnail_image` varchar(255) DEFAULT NULL,
`thumbnail_image_en` varchar(255) DEFAULT NULL,
`display_date` tinyint(4) NOT NULL,
`display_thumbnail_image` tinyint(4) NOT NULL,
`display_short_text` tinyint(4) NOT NULL,
`display_more_button` tinyint(4) NOT NULL,
`imported_from_old_site` tinyint(4) NOT NULL,
`slug` varchar(255) NOT NULL,
`slug_en` varchar(255) DEFAULT NULL,
`sort_order` int(11) NOT NULL,
`visible` tinyint(4) NOT NULL,
`created_at` datetime NOT NULL,
`updated_at` datetime DEFAULT NULL,
`deleted_at` datetime DEFAULT NULL,
`deleted_by` int(11) DEFAULT NULL,
`deleted_from_ip` varchar(255) DEFAULT NULL,
`deleted_with_browser` varchar(255) DEFAULT NULL,
PRIMARY KEY (`webpage_category_id`),
KEY `Fk_User_WebpageCategory_idx` (`deleted_by`),
KEY `Fk_Parent_WebpageCategory_idx` (`parent_id`),
CONSTRAINT `Fk_Parent_WebpageCategory` FOREIGN KEY (`parent_id`) REFERENCES `webpage_category` (`webpage_category_id`) ON DELETE SET NULL ON UPDATE NO ACTION,
CONSTRAINT `Fk_User_WebpageCategory` FOREIGN KEY (`deleted_by`) REFERENCES `user` (`user_id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mb3;
Since I use soft delete in PHP, I want when a specific webpage category is deleted all rows that have set that webpage category as parent_id,those parent_id fields should be set to null.
I have the following code:
DELIMITER //
CREATE TRIGGER _set_webpage_category_foreign_key_to_null
AFTER UPDATE ON webpage_category
FOR EACH ROW
BEGIN
IF !(NEW.deleted_at <=> OLD.deleted_at) THEN
update webpage_category
set parent_id = NULL
WHERE parent_id = NEW.webpage_category_id;
END IF;
END; //
The error that I get in log file is: Can't update table 'webpage_category' in stored function/trigger because it is already used by statement which invoked this stored function/trigger
Any help will be deeply appreciated.