pocketmine\level\Level::unregisterGenerator PHP Method

    public function unregisterGenerator()
    {
        $size = $this->server->getScheduler()->getAsyncTaskPoolSize();
        for ($i = 0; $i < $size; ++$i) {
            $this->server->getScheduler()->scheduleAsyncTaskToWorker(new GeneratorUnregisterTask($this, $this->generatorInstance), $i);
        }
    }