Kirby 3: The form could not be saved, check your API connection

I am getting this same error, but only when adding hooks. I am testing with code straight from docs:

'hooks' => [
    'page.create:before' => function ($event) {
        var_dump($event->name());   // 'page.create:before'
        var_dump($event->type());   // 'page'
        var_dump($event->action()); // 'create'
        var_dump($event->state());  // 'before'
        var_dump($event->page());   // $page
        var_dump($event->input());  // $input

If I keep the hook, but remove var_dump then the save happens correctly still. Debug mode is enabled. Any ideas?