Java.lang.NullPointerException Error

Summary

system throws Java.lang.NullPointerException Error when trying to save invoice

Content

I have a user who has entered the header of an invoice, saves, then proceeds to enter five lines containing project funding. Once completed, when the user tries to save, go into Manage Installments or Initiate the invoice, the system throws Java.lang.NullPointerException Error. The user is unable to take any action and is forced to cancel invoice. I have checked her security roles, all the projects are active, and the supplier is set up correctly. 

If anyone has any insight it would be much appreciated. This is an error we've seen for over a year, it doesn't happen often, but I'd like to get a resolution on it.