function findOrCreateSheetByName_(ss, name) {
  let sheet = ss.getSheetByName(name);
  if (sheet) {
    return sheet;
  }
  return ss.insertSheet(name);
}
Retrieves a sheet by name, creating it if it doesn't yet exist.

@param {Spreadsheet} ss - Containing spreadsheet
@Param {string} name - Name of sheet to return
@return {Sheet} Sheet instance