add null check

apps/app/src/server/routes/apiv3/page/update-page.ts

@@ -136,9 +136,8 @@ export const updatePageHandlersFactory: UpdatePageHandlersFactory = (crowi) => {
 
       // check revision
       const currentPage = await Page.findByIdAndViewer(pageId, req.user);
-      const isUpdatable = await currentPage.isUpdatable(revisionId, origin);
 
-      if (currentPage != null && !isUpdatable) {
+      if (currentPage != null && !await currentPage.isUpdatable(revisionId, origin)) {
         const latestRevision = await Revision.findById(currentPage.revision).populate('author');
         const returnLatestRevision = {
           revisionId: latestRevision?._id.toString(),