My libGDX applet question didn't get any answers. Adding a bounty also didn't get any answers. What can I do? I heard that it's possible to modify (add) more bounty, but I don't see any functionality.
Alternatively, if you want free rep, just add an answer and I'll give it to you -- it's better than letting it go to waste.
Well you've done all you can do. At this point the best thing is to try to add more detail to the question (other than posting a link to a tutorial) and trying to solve it yourself.
Not every question on the site is going to get answered. Fortunately, gamedev is one of the higher ones in terms of answered question percentages: http://stackexchange.com/sites?sort=percentanswered We're sitting at 96% of questions answered (I think they count "answered" as "having any upvoted answers" as opposed to "having an answer mark as accepted", though).
Bounties are overrated. If a question doesn't get a useful answer it's most often because not a lot of people are able to answer it, or it's phrased too poorly for people to understand it. A bounty can give you a little extra attention, but it doesn't fix the fundamental problems, and very often does it does not really make a difference. This is especially true on a small site like this where a relatively large proportion of the community will see each question anyway.
Contact the libGDX developers or fill an issue in their issue tracker. They know a lot more about libGDX than we do. I don't really know Java, but it seems like the exception gets thrown from the runtime. So you could also ask on a Java support forum or even on StackOverflow.
