"Under Law 12, a foul should be given when a player “tackles an opponent to gain possession of the ball, making contact with the opponent before touching the ball." That would suggest Johnston would be in the clear as he did appear to make contact with the ball first.
- there is a caveat if the defending player is deemed “careless, reckless, or using excessive force.”
Careless” is when the player has not exercised due caution in making his play, “reckless” means that the player has made unnatural movements designed to intimidate an opponent or to gain unfair advantage. And "excessive force” is when the player has far exceeded the use of force necessary to make a fair play for the ball and has placed his opponent in considerable danger of bodily harm.