getAutoCompleteResults isn't checking the return result of getAutoCompleteResult
getAutoCompleteResult has the type signature public function getAutoCompleteResult(Entity $entity,array $options = []): ?array;
But it is used as:
[CODE="php"] return array_filter(array_merge(
[
'id' => $result->getEntityContentTypeId(),
'type' => \XF::app()->getContentTypePhrase($type),
],
$handler->getAutoCompleteResult($result, $options)
));[/CODE]
Passing null to array_merge will cause an error.
This can happen when a thread doesn't have a first post. This can be caused by bad data, or a race between between when a thread is converted into a redirect but search hasn't been updated yet.
