Yes, I suppose that will be an issue for some. People need a reason to live...something to do, something to work towards and give them purpose. Look at retirees who worked hard their entire lives, thinking that playing golf everyday and just chillaxing will be the life they always wanted...and then die within 5 years, mostly from boredom.
So, what will need to be done is some kind of "proof of work" in order to maintain their income. Some kind of part-time job, volunteer work, student work and such. It doesn't have to be "for pay" work, but something that benefits society and keeps the individual engaged and involved.
Nothing is perfect, and there will be problems with a Universal Income. But the alternative is that millions, maybe billions, will be out of work and penniless, while the few wealthy have such colosal wealth, they could buy the world several times over, but only hoard their wealth. That kind of imbalance, the kind of imbalance we have today, cannot last. Nature itself demands balance (not necessarily "equality", but a equitable balance) and the nature in us, our human nature will demand balance too. Even if it requires violence.
Universal Income, I believe, can provide that balance without violence.