OK, the only way that can throw a null pointer exception is if supplierSessions is null. So probably, either you aren't initialising it anywhere or you are but that code isn't being called for some reason. Either way, it means the actual error isn't in the code you've posted.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com
subject: Getting java.lang.NullPointerException error with (SelectONE)