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XF\Entity\CoverImageTrait::getCoverImageInternal()

[php]

/** @var Attachment $attachment */

if ($embeddedAttachments[$attachment->attachment_id])

{

    continue;

}

[/php]

This calls \XF\Mvc\AbstractCollection::offsetGet()

[php]

public function offsetGet($key): ?Entity

{

    return $this->entities[$key];

}

[/php]

As can be seen, this directly accesses the internal array without checking if the key exists, so if it does not exist this generates a warning Undefined array key ...

It's a bit tricky to actually see this warning as \XF\Template\Templater::handleTemplateError() hides it so this needs to be called by code outside of templater context.