title: marker icon isn't showing in Leaflet
body: <p>I 've put together a very simple React / Leaflet demo but the marker icon is not showing at all.
  Full running code is <a href="https://codesandbox.io/s/modest-tree-7puvc?fontsize=14&amp;hidenavigation=1&amp;theme=dark" rel="noreferrer">here</a>.</p>
  
  <p>Here's what I have in my <code>componentDidMount</code> method:</p>
  
  <pre><code>componentDidMount() {
  this.map = L.map("map-id", {
    center: [37.98, 23.72],
    zoom: 12,
    zoomControl: true
  });
  
  const mapboxAccessToken =
    "pk.eyJ1IjoibXBlcmRpa2VhcyIsImEiOiJjazZpMjZjMW4wOXJzM2ttc2hrcTJrNG9nIn0.naHhlYnc4czWUjX0-icY7Q";
  L.tileLayer(
    "https://api.mapbox.com/styles/v1/{id}/tiles/{z}/{x}/{y}?access_token={accessToken}",
    {
      attribution:
        'Map data &amp;copy; &lt;a href="https://www.openstreetmap.org/"&gt;OpenStreetMap&lt;/a&gt; contributors, &lt;a href="https://creativecommons.org/licenses/by-sa/2.0/"&gt;CC-BY-SA&lt;/a&gt;, Imagery © &lt;a href="https://www.mapbox.com/"&gt;Mapbox&lt;/a&gt;',
      maxZoom: 25,
      id: "mapbox/streets-v11",
      accessToken: mapboxAccessToken
    }
  ).addTo(this.map);
  
  L.marker([37.98, 23.72])
    .addTo(this.map)
    .bindPopup("a marker")
    .openPopup();
  }
  </code></pre>
  
  <p>Basically, the popup is visible, but not the marker icon itself. I.e., here's what I see:</p>
  
  <p><a href="https://i.stack.imgur.com/Q5gdD.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Q5gdD.png" alt="enter image description here"></a></p>