successCallback of fullCalendar v5.11.0 is not returned

Question

I am having trouble because the successCallback of fullCalendar v5.11.0 may not return. I would like to know how to solve the problem.

I am using fullCalendar v5.11.0, I am creating a web application that displays data obtained from Google Calendar.

The app is working without any problems, but In rare cases, the page freezes when displayed.

After calling successCallback in the code shown below, it freezes.

function onEvents(fetchInfo, successCallback, failureCallback) {
  $.ajax({
    url: '/user/ajax_get_calendar/',
    type: 'GET',
    data: {
      start: fetchInfo.startStr,
      end: fetchInfo.endStr,
    },
    dataType: 'json',
  }).done(response => {
    successCallback(response.events);
  });
}