NearBylocation issue

Ionic Framework ionic-v3
1

I try to implement find nearby location.but there is a problem map is showing but nearby markar is not showing please guide me.
below my code.

constructor(public navCtrl: NavController,private ngZone: NgZone, private geolocation : Geolocation) {}

  ionViewDidLoad() {
    console.log('ionViewDidLoad DetailsPage');
    this.loadMap();

  }

  @ViewChild('map') mapElement: ElementRef;
  map: any;

  loadMap(){

    this.geolocation.getCurrentPosition().then((position) => {

      let latLng = new google.maps.LatLng(position.coords.latitude, position.coords.longitude);

      let mapOptions = {
        center: latLng,
        zoom: 15,
        mapTypeId: google.maps.MapTypeId.ROADMAP
      }

      this.map = new google.maps.Map(this.mapElement.nativeElement, mapOptions);

      let service = new google.maps.places.PlacesService(mapOptions);

      service.nearbySearch({
        location: latLng,
        radius: 500,
        type: ['pub']
      }, (results, status) => {
          this.callback(results, status)
           this.addMarker();
      });

    }, (err) => {
      console.log(err);
    });

  }
  callback(results, status) {
    if (status === google.maps.places.PlacesServiceStatus.OK) {
      for (var i = 0; i < results.length; i++) {
        this.createMarker(results[i]);
      }
    }
  }
  createMarker(place){
    var placeLoc = place.geometry.location;
    var marker = new google.maps.Marker({
        map: this.map,
        position: place.geometry.location
    });

    let infowindow = new google.maps.InfoWindow();

    google.maps.event.addListener(marker, 'click', () => {
      this.ngZone.run(() => {
        infowindow.setContent(place.name);
        infowindow.open(this.map,this);
      });
    });
  }
  addMarker(){
    let marker = new google.maps.Marker({
      map: this.map,
      animation: google.maps.Animation.DROP,
      position: this.map.getCenter()
    });

    let content = "<h4>You are here!</h4>";          

    this.addInfoWindow(marker, content);

  }

  addInfoWindow(marker, content){

    let infoWindow = new google.maps.InfoWindow({
      content: content
    });

    google.maps.event.addListener(marker, 'click', () => {
      this.ngZone.run(() => {
        infoWindow.open(this.map, marker);
      });
    });
  }
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