FIX: use const instead of let for observedCache

assets/javascripts/discourse/mixins/undo-changes.js.es6

@@ -4,7 +4,7 @@ import { get, set } from "@ember/object";
 import Mixin from "@ember/object/mixin";
 import { deepEqual } from "discourse-common/lib/object";
 
-let observedCache = [];
+const observedCache = [];
 
 export default Mixin.create({
   didInsertElement() {
@@ -36,7 +36,10 @@ export default Mixin.create({
     listProperties(componentType, opts).forEach((property) => {
       if (observedCache.includes(property)) {
         obj.removeObserver(property, this, this.toggleUndo);
-        observedCache = observedCache.filter((p) => p !== property);
+        let index = observedCache.indexOf(property);
+        if (index !== -1) {
+          observedCache.splice(index, 1);
+        }
       }
     });
   },