Scheduler - The "Cannot read property 'option' of undefined" error occurs on an attempt to open the appointment popup for the second time if a form item is hidden in the onAppointmentFormOpening event handler

We checked this scenario and found that it is not correct to hide the entire form item in this scenario. Let me describe the situation in greater detail. We create the appointment form only once, when it is opened for the first time. Then, we work with the existing form. So, when you hide a form item, it may break the internal logic of other form items. For example, in this particulate case, the startDate and endDate items depend on each other, so when the endDate item is hidden, the issue occurs.
In such scenarios when you need to hide default items, the best way is to create the appointment form from scratch, so you won't depend on the implementation/limitations of the default form.

However, if you need to hide only one item, you can use the initial workaround:

JavaScript
function onAppointmentFormOpening(data) {
  const form = data.form;
  form.itemOption("mainGroup.endDate", "label", { visible: false });
  form.getEditor("endDate").option("visible", false);
}

For some frameworks this workaround may differ and require additional adjustments in complex scenarios. For example:
Vue:

JavaScript
onAppointmentFormOpening(data) {
      const form = data.form;
      this.$nextTick(() => {
        form.itemOption("mainGroup.endDate", "label", { visible: false });
        form.getEditor("endDate").option("visible", false);
      });
}

We will add the information about this limitation to our documentation in the future.