Storm's attraction / affection towards Null is not something that can be considered out of the blue
Throughout the course of the game she observes him taking a stance to help his fellow mutants , be it during the juggernaut incident or him basically sacrificing himself so that the other mutant students could escape the sentinels attacking the mall , it's his actions that made him enduring to her , she even tries to visit him to make sure he is recovering well but feels ashamed with herself since she couldn't be there for him
Overall there is a genuinely good build up as to why she went in for that kiss , the Shadow King's mind control only amplified her true emotions that she had been trying to suppress because of obvious ethical reasons as her being his teacher , but at that one moment she let go of those inhibitions and her true feelings came forward