Dropdown fields in GForms using AppScript

I'm trying to control my GForms using AppScript and would like to delete and fill the dropdown fields using a script. I've written the following scripts for this and stored them in the GForms file. The scripts work without an error message, but I can see success in the form. I've made a GForms publicly available here: Link

Who has a tip for me where I made a mistake?

Thanks in advance.

For Delete

function deleteDropdownValues() {
  var formId = "1zf8tNDLHQZwFQnLWTFYTrx_X4k_ESLrM5kqUdCqz6GA";
  var itemId = "2015071815"; // ID des Dropdown-Feldes

  var form = FormApp.openById(formId);
  var item = form.getItemById(itemId);

  if (item.getType() === FormApp.ItemType.MULTIPLE_CHOICE) {
    item.asMultipleChoiceItem().setChoices([]);
    Logger.log('Dropdown-Werte erfolgreich gelöscht.');
  } else {
    Logger.log('Das angegebene Item ist kein Dropdown-Element.');
  }
}

To add the values to the DropDown-Field

function addOptionToDropdown() {
  var formId = "1zf8tNDLHQZwFQnLWTFYTrx_X4k_ESLrM5kqUdCqz6GA";
  var itemId = "2015071815"; // ID des Dropdown-Feldes
  var newOption = "XXXX";

  // Form and Item abrufen
  var form = FormApp.openById(formId);
  var item = form.getItemById(itemId);

  // Add new values
  item.asListItem().createChoice(newOption);
  item.asListItem().createChoice("sssssss");
  item.asListItem().createChoice("eeeeeeeee");
}

Use LIST and setChoiceValues

You may use this modified version of your script that should work with what you'd like to do.

For deleting drop-down values:

function deleteDropdownValues() {
  var formId = "1zf8tNDLHQZwFQnLWTFYTrx_X4k_ESLrM5kqUdCqz6GA";
  var itemId = "2015071815";

  var form = FormApp.openById(formId);
  var item = form.getItemById(itemId);

  if (item.getType() === FormApp.ItemType.LIST) {
    item.asListItem().setChoiceValues([""]);
    Logger.log('Dropdown-Werte erfolgreich gelöscht.');
  } else {
    Logger.log('Das angegebene Item ist kein Dropdown-Element.');
  }
}

I changed FormApp.ItemType.MULTIPLE_CHOICE to FormApp.ItemType.LIST since it's item type is a LIST and setChoices([]) to setChoiceValues([""]) since for some reason it's giving out errors such as Exception: Array is empty: choices and Exception: The parameters (number[]) don't match the method signature for FormApp.ListItem.setChoices.

Output:

For adding values in the drop-down field:

function addOptionToDropdown() {
  var formId = "1zf8tNDLHQZwFQnLWTFYTrx_X4k_ESLrM5kqUdCqz6GA";
  var itemId = "2015071815"; // ID des Dropdown-Feldes
  var newOption = ["Option 1", "sssssss", "eeeeeeeee"];

  // Form and Item abrufen
  var form = FormApp.openById(formId);
  var item = form.getItemById(itemId);

  // Add new values
  item.asListItem().setChoiceValues(newOption);
}

I changed the createChoice()'s to setChoiceValues(newOption) on this script and you may just put your options in the newOption array.

Output: