GAS - SpreadsheetApp.openById get always NULL

I trying to get access to another spreadsheet than the active one.

Then, first, I tried to get access to a spreadsheet.

I have spent hours, but I still do not obtain other value than Null of :

var ss = SpreadsheetApp.openById("xxxxxxxxxxx");
Logger.log(ss.getName());

event if I set xxxx as the Id of the active spreadsheet.

To be more precise, my code is the following :

var ss123 = SpreadsheetApp.openById('xxxxxxxxx');
  Logger.log("Name of file  : ",ss123.getName());

and the result :

23:56:21    Infos   Name of file : 

Thanks for your help !