Uncaught TypeError: this._map is null (Vue.js 3, Leaflet)

I am getting a strange error from Leaflet in a Vue.js project (version 3).

If I close a popup and zoom in/out, this error occurs on Firefox:

Uncaught TypeError: this._map is null

And on Chrome:

Cannot read property '_latLngToNewLayerPoint' of null

The map component is as follows:

<template>
  <div id="map"></div>
</template>

<script>
import "leaflet/dist/leaflet.css";
import L from 'leaflet';

export default {
  name: 'Map',
  data() {
    return {
      map: null
    }
  },
  mounted() {
    this.map = L.map("map").setView([51.959, -8.623], 12);
    L.tileLayer("https://{s}.tile.osm.org/{z}/{x}/{y}.png", {
        attribution: '&copy; <a href="http://osm.org/copyright">OpenStreetMap</a> contributors'
    }).addTo(this.map);

    L.circleMarker([51.959, -8.623]).addTo(this.map)
      .bindPopup('I am a marker')
      .openPopup();
  }
}
</script>

<style scoped>
  #map {
    height: 300px;
    width: 100%;
  }
</style>

How to reproduce the error:

1. Open stackblitz: https://stackblitz.com/edit/vue-gjeznj
2. Close popup
3. Zoom in/out

Can it be just a bug? Or is there any error in code that I missed?

FWIW, this seems a new issue since Vue 3.

The problem is absent from Vue version 2 with Leaflet: https://codesandbox.io/s/fast-firefly-lqmwm?file=/src/components/HelloWorld.vue

Just to make sure, here is a reproduction of the issue with the same code but Vue version 3, on CodeSandbox: https://codesandbox.io/s/laughing-mirzakhani-sgeoq?file=/src/components/HelloWorld.vue

What seems to be the culprit is the proxying of this.map by Vue, which seems to interfere with Leaflet events (un)binding. It looks like Vue 3 now automatically performs deep proxying, whereas Vue 2 was shallow.

As described in https://v3.vuejs.org/api/basic-reactivity.html#markraw:

[...] the shallowXXX APIs below allow you to selectively opt-out of the default deep reactive/readonly conversion and embed raw, non-proxied objects in your state graph. They can be used for various reasons:

• Some values simply should not be made reactive, for example a complex 3rd party class instance, or a Vue component object.

...which is the case of Leaflet built map object.

A very simple workaround would be not to use this.map (i.e. not to store the Leaflet built map object in the component state, to prevent Vue from proxying it), but to just store it locally (e.g. const map = L.map() and then myLayer.addTo(map)).

But what if we do need to store the map object, typically so that we can re-use it later on, e.g. if we want to add some Layers on user action?

Then make sure to properly unwrap / unproxy this.map before using it with Leaflet, e.g. using Vue 3 toRaw utility function:

Returns the raw, original object of a reactive or readonly proxy. This is an escape hatch that can be used to temporarily read without incurring proxy access/tracking overhead or write without triggering changes.

import { toRaw } from "vue";

export default {
  name: "Map",
  data() {
    return {
      map: null,
    };
  },
  mounted() {
    const map = L.map("map").setView([51.959, -8.623], 12);
    L.tileLayer("https://{s}.tile.osm.org/{z}/{x}/{y}.png", {
      attribution:
        '&copy; <a href="http://osm.org/copyright">OpenStreetMap</a> contributors',
    }).addTo(map);

    L.circleMarker([51.959, -8.623])
      .addTo(map)
      .bindPopup("I am a marker")
      .openPopup();

    this.map = map;
  },
  methods: {
    addCircleMarker() {
      L.circleMarker([
        51.959 + Math.random() * 0.05,
        -8.623 + Math.random() * 0.1,
      ])
        .addTo(toRaw(this.map)) // Make sure to "unproxy" the map before using it with Leaflet
        .bindPopup("I am a marker")
        .openPopup();
    },
  },
}

Demo: https://codesandbox.io/s/priceless-colden-g7ju9?file=/src/components/HelloWorld.vue