Bump Leaflet to 1.9.4

Changelog:

https://github.com/Leaflet/Leaflet/blob/main/CHANGELOG.md

Sounds safe for us. Tested localy. Tests pass. Inch Allah.

package.json

@@ -36,7 +36,7 @@
     "csv2geojson": "5.1.1",
     "dompurify": "^3.0.3",
     "georsstogeojson": "^0.1.0",
-    "leaflet": "1.3.4",
+    "leaflet": "1.9.4",
     "leaflet-contextmenu": "^1.4.0",
     "leaflet-editable": "^1.2.0",
     "leaflet-editinosm": "0.2.3",

umap/static/umap/js/umap.layer.js

@@ -1060,7 +1060,7 @@ L.U.DataLayer = L.Evented.extend({
   },
 
   isVisible: function () {
-    return this.map.hasLayer(this.layer)
+    return this.layer && this.map.hasLayer(this.layer)
   },
 
   canBrowse: function () {