This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
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.