KNOW YOUR LAWS:
Can you tell why this was the right call?
If a player jumps and catches the ball, both feet must land in the playing area otherwise the ball is in touch or touch-in-goal. The mistake Nadolo makes is that he holds the ball for a split second whilst it has crossed the plane of the line and then one of his feet then lands in touch, so the linesman in fact makes an excellent call. If he had knocked or palmed the ball back it would be fine, but crucially he carries it.