The popcorn may have been a defensive move when the old guy pulled his gun.
I thought about that and it is entirely possible.
"Reeves claims he feared for his safety after being hit by an unknown object".
-- but no "unknown object" was found.".
Would you describe a bag of popcorn as an "unknown object"???
Popcorn is a pretty well known object in a movie theatre especially when you are only a few feet away from it.
It's pretty clear from his statement that that bag of popcorn thrown or not was not a factor in the shooting -- the unknown object that didn't exist was.