iOS6模拟器MKMapKit“在框架中找不到default.styleproto”

问题描述：iOS6模拟器MKMapKit“在框架中找不到default.styleproto”

swift
import UIKit
import MapKit
class ViewController: UIViewController {
    @IBOutlet weak var mapView: MKMapView!
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 创建一个坐标
        let coordinate = CLLocationCoordinate2D(latitude: 37.331686, longitude: -122.030656)
        
        // 创建一个区域范围
        let span = MKCoordinateSpan(latitudeDelta: 0.1, longitudeDelta: 0.1)
        let region = MKCoordinateRegion(center: coordinate, span: span)
        
        // 设置地图显示区域
        mapView.setRegion(region, animated: true)
        
        // 创建一个标注
        let annotation = MKPointAnnotation()
        annotation.coordinate = coordinate
        annotation.title = "Apple Inc."
        annotation.subtitle = "Cupertino, CA"
        
        // 添加标注到地图上
        mapView.addAnnotation(annotation)
    }
}