I have some problems with the GPS Signal Map on my Leaflet Map. I have now a Map without GPS Signal. I try to copy the Code lines from the tutorial of leaflet.com, but it doesn't work :(

Can anyone help me?

```html
<!DOCTYPE html>
<html>
<head>

 <title>Layers Control Tutorial - Leaflet</title>

 <meta charset="utf-8" />
 <meta name="viewport" content="width=device-width, initial-scale=1.0">

 <link rel="shortcut icon" type="image/x-icon" href="docs/images/favicon.ico" />

    <link rel="stylesheet" href="https://unpkg.com/leaflet@1.1.0/dist/leaflet.css" integrity="sha512-wcw6ts8Anuw10Mzh9Ytw4pylW8+NAD4ch3lqm9lzAsTxg0GFeJgoAtxuCLREZSC5lUXdVyo/7yfsqFjQ4S+aKw==" crossorigin=""/>
    <script src="https://unpkg.com/leaflet@1.1.0/dist/leaflet.js" integrity="sha512-mNqn2Wg7tSToJhvHcqfzLMU6J4mkOImSPTxVZAdo+lcPlk+GhZmYgACEe0x35K7YzW1zJ7XyJV/TT1MrdXvMcA==" crossorigin=""></script>

 <style>
  #map {
   width: 600px;
   height: 400px;
  }
 </style>

</head>
<body>

<div id='map'></div>

<script>
 var citie = L.layerGroup();

 L.marker([52.524491, 13.192954]).bindPopup('This is Littleton, CO.').addTo(citie),
 L.marker([52.524273, 13.196197]).bindPopup('This is Denver, CO.').addTo(citie),
 L.marker([52.525986, 13.196685]).bindPopup('This is Aurora, CO.').addTo(citie),
 L.marker([52.529328, 13.195698]).bindPopup('This is Golden, CO.').addTo(citie);
 L.marker([52.527820, 13.195513]).bindPopup('This is Golden, CO.').addTo(citie);

 var um = L.layerGroup();

 L.marker([52.526729, 13.194802]).bindPopup('This is Golden, CO.').addTo(um);

 var mbAttr = 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, ' +
   '<a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, ' +
   'Imagery © <a href="http://mapbox.com">Mapbox</a>',
  mbUrl = 'https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token=pk.eyJ1IjoibWFwYm94IiwiYSI6ImNpejY4NXVycTA2emYycXBndHRqcmZ3N3gifQ.rJcFIG214AriISLbB6B5aw';

 var grayscale   = L.tileLayer(mbUrl, {id: 'mapbox.light', attribution: mbAttr}),
  streets  = L.tileLayer(mbUrl, {id: 'mapbox.streets',   attribution: mbAttr});

 var map = L.map('map', {
  center: [52.526273, 13.194531],
  zoom: 10,
  layers: [grayscale, citie]
 });

 var baseLayers = {
  "Grayscale": grayscale,
  "Streets": streets
 };

 var overlays = {
  "Haltestellenaufhebungen": citie,
      "test": um,

 };

 L.control.layers(baseLayers, overlays).addTo(map);

</script>
</body>
</html>
```