Issue title: draggable marker example: TypeError: Cannot read property 'getLatLng' of undefined
 Issue body: ## Bug report

**Before** opening an issue, make sure to read the [contributing guide](https://github.com/PaulLeCam/react-leaflet/blob/master/CONTRIBUTING.md) and understand this is a bug tracker, not a support platform.

Please make sure to check the following boxes before submitting an issue.\
**Issues opened without using this template will be closed unless they have a good reason not to follow this template.**

- [x ] All peer dependencies are installed: React, ReactDOM and Leaflet.
- [x] Using a supported version of React and ReactDOM (v16.8.0 minimum).
- [x ] Using the supported version of Leaflet (v1.6.0 minimum) and its corresponding CSS file is loaded.
- [x] Using the [latest version of React-Leaflet](https://github.com/PaulLeCam/react-leaflet/releases).
- [x] The issue has not already been reported.
- [x] Make sure you have followed the [quick start guide](http://leafletjs.com/examples/quick-start.html) for Leaflet.
- [ x] Make sure you have fully read the [documentation](https://react-leaflet.js.org/docs/en/intro.html) and that you understand the [limitations](https://react-leaflet.js.org/docs/en/intro.html#limitations).

### Expected behavior

Draggable marker in example should not fail with undefined error on completion of drag.
https://github.com/PaulLeCam/react-leaflet/blob/master/example/components/draggable-marker.js

### Actual behavior

Map loads ok and then throws error on completion of first drag

error:
```
TypeError: Cannot read property 'getLatLng' of undefined
NewClass.DraggableExample.updatePosition
src/Draggable.tsx:38
  35 |   const marker = this.refmarker.current
  36 |   if (marker != null) {
  37 |     this.setState({
> 38 |       marker: marker.leafletElement.getLatLng(),
     | ^  39 |     })
  40 |   }
  41 | }
```
### Steps to reproduce
Copy and past the example code, export component and display in parent component.
Observe map loads correctly in the browser.
Drag the pointer.
Observe the error message.

Please provide the simplest example possible to reproduce the issue, based on [this CodePen](https://codepen.io/PaulLeCam/pen/gzVmGw).

 Comments: 
 Comment 0: problem with react-leaflet-universal
{'source': 'GithubIssues', 'repo_name_with_owner': 'PaulLeCam/react-leaflet', 'issue_id': 572214438, 'repo_stars': 4466.0}