Crash when unmounting leaflet map in React: "TypeError: Cannot read properties of undefined (reading '_zoom')"

[tmadeira]

This package is some times crashing when used inside React:

TypeError: Cannot read properties of undefined (reading '_zoom')
    at i.addLayer (MarkerClusterGroup.js:127:33)
    at i._moveChild (MarkerClusterGroup.js:721:8)
    at i._childMarkerMoved (MarkerClusterGroup.js:708:9)
    ...

(I used "some times" because it doesn't always happen; it might depend on a race condition)

The crash seems to be preventable with:

diff --git a/src/MarkerClusterGroup.js b/src/MarkerClusterGroup.js
index 87c992e..c75b306 100644
--- a/src/MarkerClusterGroup.js
+++ b/src/MarkerClusterGroup.js
@@ -123,10 +123,8 @@ export var MarkerClusterGroup = L.MarkerClusterGroup = L.FeatureGroup.extend({
                //Work out what is visible
                var visibleLayer = layer,
                    currentZoom = this._zoom;
-               if (layer.__parent) {
-                       while (visibleLayer.__parent._zoom >= currentZoom) {
-                               visibleLayer = visibleLayer.__parent;
-                       }
+               while (visibleLayer.__parent && visibleLayer.__parent._zoom >= currentZoom) {
+                       visibleLayer = visibleLayer.__parent;
                }

                if (this._currentShownBounds.contains(visibleLayer.getLatLng())) {

[tmadeira]

That fix was also suggested by @adrianlzt in this PR from 2022: #1048