FIX: add observed cache to undo changes mixin

Prevents observer add / remove race conditions which can cause exceptions.

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

@@ -4,6 +4,8 @@ import { get, set } from "@ember/object";
 import Mixin from "@ember/object/mixin";
 import { deepEqual } from "discourse-common/lib/object";
 
+var observedCache = [];
+
 export default Mixin.create({
   didInsertElement() {
     this._super(...arguments);
@@ -32,7 +34,10 @@ export default Mixin.create({
     };
 
     listProperties(componentType, opts).forEach((property) => {
-      obj.removeObserver(property, this, this.toggleUndo);
+      if (observedCache.includes(property)) {
+        obj.removeObserver(property, this, this.toggleUndo);
+        observedCache = observedCache.filter(p => p !== property);
+      }
     });
   },
 
@@ -45,6 +50,9 @@ export default Mixin.create({
     };
 
     listProperties(componentType, opts).forEach((property) => {
+      if (observedCache.indexOf(property) === -1) {
+        observedCache.push(property);
+      }
       obj.addObserver(property, this, this.toggleUndo);
     });
   },

plugin.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 # name: discourse-custom-wizard
 # about: Forms for Discourse. Better onboarding, structured posting, data enrichment, automated actions and much more.
-# version: 2.2.6
+# version: 2.2.7
 # authors: Angus McLeod, Faizaan Gagan, Robert Barrow, Keegan George, Kaitlin Maddever
 # url: https://github.com/paviliondev/discourse-custom-wizard
 # contact_emails: development@pavilion.tech