Google script error: An unexpected error occurred from openById - google-apps-script

Question

I' making automation system that google spreadsheet convert it to PDF file and save on my google drive.
I've known this script works successfully on my project page, but doesn't work on trigger.
how could I resolve this problem?
the error message had occurred at 5:39 line.
here
var ss = (optSSId) ? SpreadsheetApp.openById(optSSId) : SpreadsheetApp.getActiveSpreadsheet();
Please help me!
function savePDFs( optSSId, optSheetId ) {
// If a sheet ID was provided, open that sheet, otherwise assume script is
// sheet-bound, and open the active spreadsheet.
var ss = (optSSId) ? SpreadsheetApp.openById(optSSId) : SpreadsheetApp.getActiveSpreadsheet();
// Get folder containing spreadsheet, for later export
var parents = DriveApp.getFileById(ss.getId()).getParents();
if (parents.hasNext()) {
var folder = parents.next();
}
else {
folder = DriveApp.getRootFolder();
}
//additional parameters for exporting the sheet as a pdf
var url_ext = 'export?exportFormat=pdf&format=pdf' //export as pdf
// Print either the entire Spreadsheet or the specified sheet if optSheetId is provided
+ (optSheetId ? ('&gid=' + sheet.getSheetId()) : ('&id=' + ss.getId()))
// following parameters are optional...
+ '&size=letter' // paper size
+ '&portrait=true' // orientation, false for landscape
+ '&fitw=true' // fit to width, false for actual size
+ '&sheetnames=false&printtitle=false&pagenumbers=false' //hide optional headers and footers
+ '&gridlines=false' // hide gridlines
+ '&fzr=false'; // do not repeat row headers (frozen rows) on each page
var options = {
headers: {
'Authorization': 'Bearer ' + ScriptApp.getOAuthToken()
}
}
var response = UrlFetchApp.fetch("https://docs.google.com/spreadsheets/" + url_ext, options);
var blob = response.getBlob().setName(ss.getName() + '.pdf');
//from here you should be able to use and manipulate the blob to send and email or create a file per usual.
//In this example, I save the pdf to drive
folder.createFile(blob);
}

Answer 1

If the id passed to openById() is not correct(sometimes without reason),
An unexpected error occurred from openById
is thrown, when using SpreadsheetApp.openById().
If the script is bound, you could just use var ss = SpreadsheetApp.getActiveSpreadsheet().

Answer 2

In Google Apps Scripts triggers pass an event object to the function that is being called by them.
If your trigger is calling directly the savePDFs function, then the event object is assigned to optSSId. As this is not a string, this might be the cause of the problem.
Instead of calling savePDFs directly, set your trigger to call another function, then make this fuction call savePDFs without passing any variable. I.E.:
function respondToTimeDrivenTrigger(e){
savePDFs();
}