This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
Probably because you defined each button twice. Once as a class field and once as a local field. If this is the case the button referenced in the ActionListener will be null. The solution is to not define the local field:
For more help you should create a SSCCE, that demonstrates the incorrect behaviour.