fix(datebox): isStub && Update test

Author: marker dao ®

packages/devextreme/js/__internal/ui/date_box/date_box.mask.ts

@@ -1,4 +1,3 @@
-/* eslint-disable class-methods-use-this */
 import eventsEngine from '@js/common/core/events/core/events_engine';
 import { addNamespace, isCommandKeyPressed, normalizeKeyName } from '@js/common/core/events/utils/index';
 import { getFormat } from '@js/common/core/localization/ldml/date.format';
@@ -503,7 +502,7 @@ class DateBoxMask extends DateBoxBase {
 
     const activePartIndex = this._activePartIndex ?? 0;
     let index = fitIntoRange(activePartIndex + step, 0, this._dateParts.length - 1);
-    if (this._dateParts[index].isStub) {
+    if (this._dateParts[index]?.isStub) {
       const isBoundaryIndex = (index === 0 && step < 0)
         || (index === this._dateParts.length - 1 && step > 0);
       if (!isBoundaryIndex) {

packages/devextreme/testing/tests/DevExpress.ui.widgets.editors/datebox.mask.tests.js

@@ -1302,7 +1302,8 @@ module('Date AM/PM Handling', setupModule, () => {
 
             this.instance.option({
                 value: new Date('10/10/2012 22:00'),
-                displayFormat: 'a'
+                displayFormat: 'a',
+                useMaskBehavior: true,
             });
 
             assert.strictEqual(this.$input.val(), pmName, 'initial value is localized PM');