Cheesecakes tend to crack for two main reasons. The first is overbaking them. The second is overmixing the batter. Turn the oven off and open the door right the edges are set and the middle of the bars is still slightly wobbly. And when mixing the batter, don't go full-speed with the electric mixer once you start adding the eggs. Use low speed to incorporate less air into the cheesecake batter. That said, a few cheesecake cracks aren't the worst thing in the world and it's common to see a couple in a swirled cheesecake along the lines where the two different batter flavors combine. Unless you've baked the living daylights out of your cheesecake, these bars are still going to taste delicious even with a few cracks.