The error is thrown due to the fact that the form_validator variable is referencing the jQuery wrapped DOM element not the Validator widget, thus it does not have validate method. In order to get reference to the widget you may use the getKendoValidator method:
form_validator = $(
Also note that as the Validator is attached to a form element, the validation will be executed automatically when the form is submitted, which in this case is when the button is clicked. Therefore, there is no need to manually call the validate method.
If you want to execute some custom logic, for example show an alert to the user, when form is validated you may use the validate event
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