fallback to excalidraw.com if no origin header present

src/index.ts

   handlePreflightRequest: function (req, res) {
     var headers = {
       "Access-Control-Allow-Headers": "Content-Type, Authorization",
-      "Access-Control-Allow-Origin": req.header ? req.header.origin : "https://excalidraw.com",
+      "Access-Control-Allow-Origin": (req.header && req.header.origin) || "https://excalidraw.com",
       "Access-Control-Allow-Credentials": true,
     };
     res.writeHead(200, headers);