It seems almost certain that nuclear winter is not an existential risk in and of itself but it could precipitate a civilizational collapse from which it's impossible to recover (e.g. because we've already depleted too much of the low hanging natural resource supply). This seems quite unlikely, maybe the chance conditional on nuclear winter is between 1 and 10 percent. Given that governments already consider nuclear war to be a national security threat and that the probability seems much lower than x-risk due to future technologies it seems best to focus on other things. Even if nothing direct can be done about x-risk from future technologies, movement building seems better than nuclear risk reduction.