FIX Return null if OwnerClassName doesn't exist
FIX Return null if OwnerClassName doesn't exist

Fixes an issue where elements won't load if they belong to a page type that has been removed.
* Return null if OwnerClassName doesn't exist
* Update src/Models/ElementalArea.php
Co-authored-by: Guy Marriott <ffbae69c7a458d37279be47d195c327a58919bd0@gmail.com>

dnadesign/silverstripe-elemental,dnadesign/silverstripe-elemental