Fix the network plugin crashes when Add Route button is pressed (#1281)

Summary:
Changelog:
Fix Network Mock Dialog crashes when 'Add Route' button is pressed  https://github.com/facebook/flipper/issues/1280

Pull Request resolved: https://github.com/facebook/flipper/pull/1281

Reviewed By: cekkaewnumchai

Differential Revision: D22151908

Pulled By: passy

fbshipit-source-id: cafd2b53d76cd9ea213f8be09866222a6330d043

desktop/plugins/network/index.tsx

@@ -216,7 +216,7 @@ export default class extends FlipperPlugin<State, any, PersistedState> {
         routes: routes,
         isMockResponseSupported: result,
         showMockResponseDialog: false,
-        nextRouteId: routes.length,
+        nextRouteId: Object.keys(routes).length,
       });
     });