title: Extending a react Symbol(react.forward_ref) from leaflet-react
body: <p>I'm importing Popup from react-leaflet</p>
  
  <pre><code>import  { Marker, Map, Popup, TileLayer, ZoomControl, GeoJSON, ScaleControl} from 'react-leaflet'
  </code></pre>
  
  <p>and trying to extend it</p>
  
  <pre><code>class NewPopup extends Popup  {
  
      onPopupOpen = ({ popup }: { popup: LeafletElement }) =&gt; {
          if (popup === this.leafletElement) {
              console.log("Here open");
              this.onOpen()
          }
      }
  
      onPopupClose = ({ popup }: { popup: LeafletElement }) =&gt; {
          if (popup === this.leafletElement) {
              console.log("Here close");
              this.onClose()
          }
      }
  }
  </code></pre>
  
  <p>This compiles fine, but when I run it in the browser I get the error "Super expression must either be null or a function, not object" </p>
  
  <p>When I console.log the Popup object it looks like this</p>
  
  <pre><code>{$$typeof: Symbol(react.forward_ref), render: ƒ, apply: ƒ, bind: ƒ, call: ƒ, …}
  </code></pre>
  
  <p>Rather than a React component. Do I need to import it differently? </p>