Trouble when running JME 3 Tutorial (6) - Hello Materials

Hello! When I launch Hello Materials application from Ubuntu 10.04, it throws me from my session to the screen, where I can choose the user and enter again. I’ve noticed that problem is in rendering sphere. When I turn off the light (comment last 4 lines of code), it works fine, but without sphere. Tell me please, how can I solve this?

What the ubuntu’s user session issue have to do with a jme sphere xD, that issue happens with me all the time, but it’s not a jme issue.