I agree with Tippy, a crack at the top like that could spell the branch splitting there.
However, I sort of like the shape of the tree so far, so I think there may be a way to salvage the top scaffold.
If you have the time, I would build a support for the top branch, so that the branch won’t move if there is a lot of wind. I would fasten this same support to the trunk, and make sure the whole thing is anchored to the ground. An analogy might be that it stabilizes the weak scaffold like a “cast”. I might also wrap the crack on the trunk with some rubber Temflex tape to try to keep it together.
If you can keep the crack on the trunk closed, and the bark is not already started to die, there is a very good chance the crack will heal. If it heals and doesn’t break this season or next, it will probably add enough mass around the crack that it will not ever break there.
It will be a little bit extra work, but definitely doable, with a very good probability of success. I might also keep that top scaffold pruned a little heavier throughout the summer, so won’t be whipped as much in the wind, or weigh as much on the crack.
It’s not that uncommon for top scaffolds to crack if the trunk is pruned right above them. Because of that, I generally leave a small stub above the top scaffold, when I behead a tree. It keeps that top scaffold from splitting the trunk. The little stub generally dies fairly quickly, but it keeps the trunk from splitting. Then after the season, the top scaffold has grown enough I can cut the dead stub off, which is fairly small by this time, compared to the rest of the trunk and the top scaffold.
I see Scott posted ahead of me while I had already typed most of my response, so I guess we are in agreement, but I would give the scaffold some support and use the rubber tape on the crack.