function findResponseSheetForForm_(ss) {
  let formUrl = ss.getFormUrl();
  if (!ss || !formUrl) {
    return undefined;
  }
  let sheets = ss.getSheets();
  for (let i in sheets) {
    if (sheets[i].getFormUrl() === formUrl) {
      return sheets[i];
    }
  }
  return undefined;
}
Locates the sheet containing the form responses.

@param {Spreadsheet} ss - Spreadsheet instance to search
@return {Sheet} Sheet with form responses, undefined if not found.