First off, great work! I was about to suggest you implement it on NPC's too, depending on how it works, but you beat me to it :).
Now, it works great for the balloon, but I would suggest you change the order of execution for the NPC's if possible. Right now the disappearing option happens after the first dialogue is complete. It should happen after the last dialogue. This way you can have an NPC talk to you(or give you a task), react to what you did and then disappear, it's a small difference, but an important one I think, since it allows for extra interaction with the same NPC.
While on the topic of NPC's, there is also the thing about the "alternative dialogue" and the "yes/no dialogue". Right now if you have all 3 the execution is Dialogue 1, Choice, Dialogue 2. But in the editor it's 1,2, choice. So I propose either fix the order of execution, so the yes/no dialogue is the last to execute or change the order in the editor to make clear things go in that order. Probably the second one otherwise it may break something.
Oh and about the "yes/no dialogue"- I know I was one of the people suggesting it in the first place, but I think it turned out a bit too limiting the way it is. What I suggest is changing it to more general sign of acceptance/denial. Something like ✔ and ✖ (or 👍 and 👎), while keeping the green and red border of the buttons intact. It shouldn't brake any existing games and would open possibilities for(slightly) better dialogues and quests in future.
So to summarize:
Balloon is great!
NPC's might need a change- disappear after last dialogue.
Might want to: rearrange order of actions in the editor, change "yes/no" for "âś” / âś–"
I'm sorry if it seems I'm just waiting for you to finish one thing to hit you with another suggestion or request. I really appreciate all the work you're doing, I just want to make sure the engine is as good as it can be.