In my experience, there really isn't anything good for filling the crack in the rubber and you're just as well off to leave it. I've tried super glue, rubber cement, gorilla glue, none of it really adheres to the rubber, or it gets too brittle to do any good. If you're REALLY worried about it, you could glue a tube patch on it from the under side. I will sometimes slap a tube patch inside my tubeless tires if the same spot opens up frequently, no reason it wouldn't work for a tube setup.