Fix: await dir_exists

Author: Mariko Wakabayashi

jupyter_server/services/contents/handlers.py

@@ -180,7 +180,7 @@ async def post(self, path=''):
         if file_exists:
             raise web.HTTPError(400, "Cannot POST to files, use PUT instead.")
 
-        dir_exists = cm.dir_exists(path)
+        dir_exists = await ensure_async(cm.dir_exists(path))
         if not dir_exists:
             raise web.HTTPError(404, "No such directory: %s" % path)