I'm currently encountering an issue while customizing invoice templates in jBilling and I'm seeking your expert guidance to resolve it.

Here's a brief overview of the steps I've taken and the problem I'm facing:

1) I used the 'simple_invoice_b2b' template as a reference and edited it in JasperSoft Studio to suit our specific requirements.
2) After editing, I compiled the modified JRXML file into a Jasper file successfully.
3) I placed this modified Jasper file into the 'jbilling-community-4.1.1\jbilling\resources\designs' directory as per the discussions mentioned https://jbilling.tenderapp.com/discussions/questions/124-invoice-templates.
4) Upon testing, I encountered the following error: "There has been an internal server error: uuid=c4503d46-5ce7-4347-85e5-747d89972561, message=com.sapienter.jbilling.common.SessionInternalError: java.lang.NullPointerException."
5) The changes made to the JRXML file were minimal, mainly adding a single static text field.

Could any of you kindly assist me in troubleshooting and resolving this issue?