janhenckens

<?php
use craft\base\Element;
use craft\elements\User;
use craft\events\ModelEvent;
use yii\base\Event;

Event::on(User::class, Element::EVENT_BEFORE_SAVE, function(ModelEvent $event) {
    /** @var User $user */
    $user = $event->sender;
    if(!$user->firstName || !$user->lastName) {