DEV: make pre-initializer a bit safer

Was noticing custom fields could be null is some cases so this ensures we
never reach to it if missing

assets/javascripts/discourse-assign/pre-initializers/extend-category-for-assign.js.es6

@@ -11,6 +11,11 @@ export default {
         "custom_fields.enable_unassigned_filter",
         {
           get(fieldName) {
+            // not sure how this is possible but appears to happen in some cases
+            // in test
+            if (!this && !this.custom_fields) {
+              return false;
+            }
             return Ember.get(this.custom_fields, fieldName) === "true";
           }
         }