More information on the invoice (SKU, TAX...)

Hi ivanpg94,
I think you overcomplicate things in your patch. You could something like:

public function getSKU(): string {
  $item = $this->getOrderItem()->getPurchasedEntity();
  return $item ? $item->get('sku')->value : '';
}

This also means that you can get it from your template with {{ invoice_item.getOrderItem.getPurchasedEntity.get('sku').value }}