77804 add null check

packages/app/src/server/routes/search.js

@@ -170,7 +170,7 @@ module.exports = function(crowi, app) {
             page.lastUpdateUser = serializeUserSecurely(page.lastUpdateUser);
           }
           page.bookmarkCount = (page._source && page._source.bookmark_count) || 0;
-          page._doc.seenUserCount = page.seenUsers.length;
+          page._doc.seenUserCount = (page.seenUsers && page.seenUsers.length) || 0;
           return page;
         })
         .sort((page1, page2) => {