When passed the id of an inexisting obj, a php fatal error is triggered.
Error is in ezcontentfunctioncollection.php, line 1356

        return array( 'result' => eZContentObject::fetch( $objectID )->reverseRelatedObjectList( false, $attributeID, $groupByAttribute, $params ) );

If the fetch returns null, a method is called on a non-object => crash.

The fix should be applied to all such code in that file, and possibly through all the kernel...

Hint: grep for ')->'