Difficult in debugging app script - arrays

Question

how to resolve google app scripting error: "TypeError: Cannot call method "getRange" of null. (line 16, file "Code")" when debugging the following script:
function getEmails_(q) {
var emails = [];
var threads = GmailApp.search(q);
for (var i in threads) {
var msgs = threads[i].getMessages();
for (var j in msgs) {
emails.push([msgs[j].getBody().replace(/<.*?>/g, '\n')
.replace(/^\s*\n/gm, '').replace(/^\s*/gm, '').replace(/\s*\n/gm, '\n')
]);
}
}
return emails;
}
function appendData_(sheet, array2d) {
sheet.getRange(sheet.getLastRow() + 1, 1, array2d.length, array2d[0].length).setValues(array2d);
}
function saveEmails() {
var array2d = getEmails_("Still looking for Python Programming help");
if (array2d) {
appendData_(SpreadsheetApp.getActiveSheet(), array2d);
}
}

Answer 1

Have you set the sheet variable?
var sheet = SpreadsheetApp.getActiveSheet();
Do Logger.log(array2d); to check the data is being set in the array.
It'll be because it cant find the sheet.
I sometimes had this error and had to rename the sheet and it worked. It's strange.

Answer 2

You are trying to access getRange() on a null sheet because in your context SpreadsheetApp.getActiveSheet() return null (ie. there is no active spreadsheet)
More context could help to find a way to provide an active Spreadsheet to your function.
You can by instance open a spreadsheet with :
SpreadsheetApp.open(file)
SpreadsheetApp.openById(id)
SpreadsheetApp.openByUrl(url)
And then set it active SpreadsheetApp.setActiveSpreadsheet(newActiveSpreadsheet)
https://developers.google.com/apps-script/reference/spreadsheet/spreadsheet-app
There is other ways to make a spreadsheet active, for example when a trigger is fired (ex. notifyOnFormSubmit).
To ensure you have an active spreadsheet, you can test SpreadsheetApp.getActiveSheet() before using it :
var spreadsheet = SpreadsheetApp.getActiveSheet();
if (spreadsheet) {
Logger.log('spreadsheet url:' + spreadsheet.getUrl());
}