A recent change to the Xero UI has resulted in a breaking change, where when an Account is Deleted or Archived, it causes an unhandled exception when trying to post that item.
The issue is that the Account Number is missing from the Item in Xero Inventory. When the invoice is being created, the API throws an error as the account is blank (due to the archived / deleted account).
Refer to the Item Noted in the Batch Results (from Wise-Sync).
In Xero, click on "General Settings > Inventory Items"
Search for the item from the error:
You will notice above that the account codes are missing.
Apply the account codes which are relevant to the item being posted:
You will now be able to sync the Invoice normally.