Script Google Sheets to get a link to modify sent answers

I'm trying to copy this code from another post to be able to create links in a Google Sheets column that edit the responses from the sending of Google Forms forms.

The fact is that the script does not work for me and it gives me the following error and I do not know why.

  

TypeError: The "getRange" method of null can not be called. at getEditResponseUrls (Code: 20)

What should I change? I ask it from the utmost igorancia. Thank you very much

/*
 * Global Variables
 */

// Form URL
var formURL = 'https://docs.google.com/forms/d/form-id/viewform';
// Sheet name used as destination of the form responses
var sheetName = 'Form Responses 1';
/*
 * Name of the column to be used to hold the response edit URLs 
 * It should match exactly the header of the related column, 
 * otherwise it will do nothing.
 */
var columnName = 'Edit Url' ;
// Responses starting row
var startRow = 2;

function getEditResponseUrls(){
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName(sheetName);
  var headers = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues(); 
  var columnIndex = headers[0].indexOf(columnName);
  var data = sheet.getDataRange().getValues();
  var form = FormApp.openByUrl(formURL);
  for(var i = startRow-1; i < data.length; i++) {
    if(data[i][0] != '' && data[i][columnIndex] == '') {
      var timestamp = data[i][0];
      var formSubmitted = form.getResponses(timestamp);
      if(formSubmitted.length < 1) continue;
      var editResponseUrl = formSubmitted[0].getEditResponseUrl();
      sheet.getRange(i+1, columnIndex+1).setValue(editResponseUrl);
    }
  }
}