In an attempt to confirm my findings, I attempted to create a module to return the FavPetID.
PHP Code:
$profile = $mysidia->user->getprofile();
$owned = new OwnedAdoptable($profile->getFavpetID());
$profile->getFavpetID();
$uniqueidforpet = $profile->getFavpetID();
$texta = new Paragraph;
$texta->add(new Comment("
<br>
ID: {$profile->getFavpetID()}
"));
$moduleContainer->add($texta);
Placed in a module, it correctly returned my favourite pet, but when I set it to none caused the site to return a 500 error