No, there isn't (that I know of) a law saying one cannot yell "fire" in a theater. That said, if you did such a thing and someone was injured in any way, you could be held, at least in New York, for criminal mischief ( https://ypdcrime.com/penal.law/article145.php ) at the very least. If someone died, you could be held for manslaughter. The point is that the argument that the resultant harm of one's speech should not matter because one has the first ammendment, the right to speak freely, is specious. One has the right to do nothing, but if you see someone slip and break their neck, and you don't call emergency services, you could be done for what most states call depraved indifference to human life but in New York, we call reckless endangerment in the first degree (https://nycourts.gov/judges/cji/2-PenalLaw/120/120-25.pdf ).

Also, I'm not a lawyer either, but I did talk to some friends who are before writing about it.