AOS Service automatically stops while posting the sale order for all old TAX setups

Hello all,

Recently we have implemented GST TAX Setup (IND Localization) in our AX 2012 R3. Now while posting the sales order with the new GST tax Setups, the sale order is getting posted without any issues.

But, while posting the sale order for any previous tax setups entry, when clicking the invoice button, the Service has stopped and throws error " Service has stopped...".

On debugging, i found the error throws from SalesFormLetter class in run() method. couldn't able to find why it was throwing.

Could anyone knows the solution ?