clear httpServer value when closing server

Author: eterna2

frontend/server/app.ts

@@ -74,7 +74,11 @@ export class UIServer {
    * Stops the http server.
    */
   close() {
-    return this.httpServer && this.httpServer.close();
+    if (!!this.httpServer) {
+      this.httpServer.close();
+    }
+    this.httpServer = undefined;
+    return this;
   }
 }
 
@@ -158,7 +162,7 @@ function createUIServer(options: UIConfigs) {
         console.log('Proxied request: ', (proxyReq as any).path);
       },
       pathRewrite: pathStr =>
-      pathStr.startsWith(basePath) ? pathStr.substr(basePath.length, pathStr.length) : pathStr,
+        pathStr.startsWith(basePath) ? pathStr.substr(basePath.length, pathStr.length) : pathStr,
       target: apiServerAddress,
     }),
   );