Make sure the preview option is working on localhost without local network or internet connection.

server.js

@@ -236,9 +236,20 @@ function startBrowserSync() {
 
         const options = bs.getOptions().toJS();
 
+        const urls = {
+          devTool: options.urls.local.replace(options.port, options.proxy.url.port),
+          previewFrame: options.urls.local,
+        };
+
+        // If local network is available.
+        if (options.urls.external) {
+          urls.devTool = options.urls.external.replace(options.port, options.proxy.url.port);
+          urls.previewFrame = options.urls.external;
+        }
+
         resolve({
-          devToolUrl: options.urls.external.replace(options.port, options.proxy.url.port),
+          devToolUrl: urls.devTool,
-          previewFrameUrl: options.urls.external,
+          previewFrameUrl: urls.previewFrame,
           port: options.port
         });
       });