Error Message: Item Code 'X' is not valid

The following error message

	Error from Xero - A validation exception occurred Item code 'X' is not valid

would come up if our integration has previously exported the mentioned item to Xero which was then subsequently deleted manually via Xero's website.

To get the two systems back into sync please perform the following steps in Magento head to 
Stores > Configuration > Fooman > Fooman Connect > Xero - General 

and set Enable Reset to Yes and save the configuration

Next head to Sales > Fooman Connect > Xero Orders/Invoices

and click the button Reset Item Export Status

Any subsequent exports will now no longer be missing the above item.