Kristen Johnston revealed she wasn’t sober when she filmed her iconic “Sex and the City” window scene. “I was a hot mess,” the actress admitted on Thursday’s episode of Chelsea Handler’s “Dear Chelsea” podcast. Johnston, 58, explained that she was battling addiction when she played Carrie Bradshaw’s (Sarah Jessica Parker) old friend, socialite Lexi Featherston, on Season 6 of the hit series in 2004. During the episode, titled “Splat,” Lexi is told she can’t smoke inside a packed penthouse, so she goes on a long rant before she falls out the open window and plummets 18 stories to her death. “I was just kind of messy,” Johnston recalled….