JIRA 6.4.14 with JIRA 7.2.0 - JIRA.FormDialog issue

I used JIRA 6.4.14 and each time created JIRA.FormDialog in following manner

AJS.$(function () {
    JIRA.Dialogs.createWorkItems = new JIRA.FormDialog({
        id: "create-work-items-dialog",
        trigger: "a.create-work-items-action",
        widthClass: "large",
        ajaxOptions: JIRA.Dialogs.getDefaultAjaxOptions,
        onSuccessfulSubmit : JIRA.Dialogs.storeCurrentIssueIdOnSucessfulSubmit,
        issueMsg : 'thanks_issue_created'


below code also worked fine on JIRA 6.4.14

return _form.$form.hasClass("create-work-items-start-screen");

but in JIRA7.2.0 I am observing below issue now

TypeError: _form.$form is undefined

What is should be modified for JIRA 7.2.0 for accessing to $form field in FormDialog?