In addition to the above, which I agree with, you might also view money not as a resource, but a vote in economy. If we assume that people are motivated by money, they will still work even on UBI, because what UBI shifts is just who is voting (who is making the decisions to produce stuff).

So having an UBI is not really that different than change in demand from one set of products to another, which the economy should be able to handle even according to classical theories. But people are still rewarded for useful work even in the system with UBI, as usual. In fact, if many people decide not to work with UBI, and you do a useful job, your salary might go up, since demand from all these people for your work has gone up.