mapView: GMSMapView！nil导致应用程序崩溃(swift)

import UIKit
import GoogleMaps

class MapViewController: UIViewController {
    
    var mapView: GMSMapView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 设置Google Maps API密钥
        GMSServices.provideAPIKey("YOUR_API_KEY")
        
        // 初始化地图视图
        let camera = GMSCameraPosition.camera(withLatitude: -33.86, longitude: 151.20, zoom: 6.0)
        mapView = GMSMapView.map(withFrame: view.bounds, camera: camera)
        view.addSubview(mapView)
        
        // 添加一个标记
        let marker = GMSMarker()
        marker.position = CLLocationCoordinate2D(latitude: -33.86, longitude: 151.20)
        marker.title = "Sydney"
        marker.snippet = "Australia"
        marker.map = mapView
    }
}