Flutter:在构建Googlemap时显示Circularprogressindicator

import 'package:flutter/material.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';

class GoogleMapPage extends StatefulWidget {
  @override
  _GoogleMapPageState createState() => _GoogleMapPageState();
}

class _GoogleMapPageState extends State<GoogleMapPage> {
  GoogleMapController mapController;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Google Map'),
      ),
      body: Stack(
        children: [
          GoogleMap(
            onMapCreated: (controller) {
              setState(() {
                mapController = controller;
              });
            },
            initialCameraPosition: CameraPosition(
              target: LatLng(37.42796133580664, -122.085749655962),
              zoom: 14.0,
            ),
          ),
          if (mapController == null)
            Center(
              child: CircularProgressIndicator(),
            ),
        ],
      ),
    );
  }
}