Google App Script TypeError Cannot Call Method getSheets of null

Question

Wondering if anyone can assist in debugging this Google app script. The script works as needed to create CSV files of each Sheet in my workbook, but it throws an error every time:

"TypeError: Cannot call method "getSheets" of null. (line 10, file "Code")"

I run this on a timer trigger every 15 minutes, so I'm starting to get a lot of error messages piling up, even though the output is correct each time.

//DOWNLOAD CSVs
function onOpen() {
var ss = SpreadsheetApp.getActiveSpreadsheet();
var csvMenuEntries = [{name: "export as csv files", functionName: "saveAsCSV"}];
ss.addMenu("csv", csvMenuEntries);
};

function saveAsCSV() {
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheets = ss.getSheets();
// DriveApp.
// create a folder from the name of the spreadsheet
//var folder = DriveApp.getFoldersByName("BrandonTestFolder");
for (var i = 0 ; i < sheets.length ; i++) {
var sheet = sheets[i];
// append ".csv" extension to the sheet name
fileName = sheet.getName() + ".csv";
// convert all available sheet data to csv format
var csvFile = convertRangeToCsvFile_(fileName, sheet);
// create a file in the Docs List with the given name and the csv data
var files = DriveApp.getFilesByName(fileName);
while(files.hasNext()) {
var file = files.next();
file.setContent(csvFile)
}
//DriveApp.createFile(fileName, csvFile)
}
// Browser.msgBox('Files are waiting in a folder named ' + DriveApp.getRootFolder().getName());
}
function convertRangeToCsvFile_(csvFileName, sheet) {
// get available data range in the spreadsheet
var activeRange = sheet.getDataRange();
try {
var data = activeRange.getDisplayValues();
var csvFile = undefined;
// loop through the data in the range and build a string with the csv data
if (data.length > 1) {
var csv = "";
for (var row = 0; row < data.length; row++) {
  for (var col = 0; col < data[row].length; col++) {
if (data[row][col].toString().indexOf(",") != -1) {
data[row][col] = "\"" + data[row][col] + "\"";
}
}
// join each row's columns
// add a carriage return to end of each row, except for the last one
if (row < data.length-1) {
csv += data[row].join(",") + "\r\n";
}
else {
csv += data[row];
}
}
csvFile = csv;
}
return csvFile;
}

catch(err) {
Logger.log(err);
Browser.msgBox(err);
}
}

Answer 1

The "active" methods, like getActiveSpreadsheet(), return null when the script is triggered by a time-driven trigger. You should replace this by openById('yourSpreadsheetId')/openByUrl('yourSpreadsheetUrl') methods.