Errors triggered by correct hooks for Page Reference field

[theoretic]

Hi everybody! And thanks for Processwire!

There's a kind of bug or incorrect behavior associated with hooks for Page Reference fields.

The Page Reference field stores references for PW page(s), sorry for being so banal. This kind of field needs some rules to fetch PW pages which will be available as content of inputfield. There are some possibilities to fetch that pages:

1. by choosing their parent
2. by choosing their template
3. by custom find procedure having a nice user interface
4. by PW selector string
5. by custom PHP code

Let's now focus on the last 5th option. In this case we should add some PHP code to /site/ready.php . Just an example from PW docs:
 

$wire->addHookAfter('InputfieldPage::getSelectablePages', function($event) {
  if($event->object->hasField == 'cloth') {
    $event->return = $event->pages->find('your selector here');
  }
});

Up to this point, the things are simple and easy. But what if we will try something more complex?

...
$event->return = $event->arguments('page')->parents('template=product')->first()->findOne('name=clothes,include=all')->children('template=cloth');
...

It's an excerpt from my real hook. I tested it both outside of the hook and inside it, it works (fetches the pages i need). But when i try so edit and/or to save my Page Reference field which uses that hook, i get an error of this kind:

Fatal Error: Uncaught Error: Call to a member function find() on bool

One more time: the above-cited piece of php code is correct. It populates my inputfield with the desired page references, the pages having that pagefield can be saved, fetched and so on. The only problem is that fatal error during save/edit of pagefield itself.

I can suppose that this error takes place because the above-cited custom php code deals with some parent page(s). It works when the inputfield is used by non-admin PW page having correct parent(s) expected by cutom php code, but fails when we're trying to work with the admin PW page representing this inputfield itself.

If so, we can consider a kind of "sandbox" for custom php code of this kind. Ideally, it shouldn't give us a fatal error even if it's incorrect. It should better throw an exception and output a warning, still giving us possibility to edit/save the inputfield admin page.

Any ideas are welcome. Thanks in advance!

[theoretic]

It appears i've found a quick workaround for the problem. It's not a patch for PW core, just a methodology of writing hooks.

Okay, here's a part of real hook which works perfectly on every site/admin pages having a certain inputfield except the admin page for that inputfield:

...
$event->return = $event->arguments('page')->parents('template=products')->first()->find('template=cloth');
...

This hook fails on the inputfield admin page because first() is applied to nothing in this case. It's rather logical: the inputfield admin page has no parent with template=product ! So we need to split the above cited code into parts and to add some logic which will prevent the hook error on our inputfield admin page:

...
//$realProductsPage works everywhere except the inputfield admin page, it fails there because admin page has no parent with template=products 
$realProductsPage = $event->arguments('page')->parents('template=products')->first();

//$stubProductsPage is used only on the inputfield admin page to prevent hook error
$stubProductsPage = $event->pages->findOne('template=products');

$productsPage = $realProductsPage? : $stubProductsPage;

$event->return = $productsPage->find('template=cloth');
...

It's far from being a perfect solution but it works.