fixed: exception occurs when opening a discrepancy note

test/client/form-extensions.spec.js

@@ -34,7 +34,11 @@ describe('Extended Form Class', () => {
         expect(form.evaluationCascadeAdditions.length).to.equal(2);
     });
 
-    it('has multiple invalid constraint classes', () => {
+    it('so that Form instance has multiple invalid constraint classes', () => {
         expect(form.constraintClassesInvalid.length).to.be.above(1);
     });
+
+    it('so that Form prototype has multiple invalid constraint classes', () => {
+        expect(Form.prototype.constraintClassesInvalid.length).to.be.above(1);
+    });
 });

widget/discrepancy-note/dn-widget.js

@@ -974,7 +974,8 @@ class Comment extends Widget {
 
             return el ? el.textContent : '';
         }
-        return Form.constraintClassesInvalid
+
+        return Form.prototype.constraintClassesInvalid
             .map((invalidClass) => {
                 if (this.linkedQuestion.classList.contains(invalidClass)) {
                     const el = this.linkedQuestion.querySelector(