TypeError: Cannot read property 'getSheetByName' of null (line 3, file "Code") [closed]

I'm running a script below to send email automatically from google sheets using the cell value as the trigger. But I faced this error message when test running the script. Any idea why? thanks

TypeError: Cannot read property 'getSheetByName' of null (line 3, file "Code")

function SendEmail_AutoMedia_nic() {  // Fetch the target value  var MyValuesRange = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Master").getRange("H6");   var MyValues = MyValuesRange.getValue();  // Check the target value  if (MyValues > 0){  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("email list")  var startRow = 2; // First row of data to process  var numRows = 10; // Number of rows to process  // Fetch the range of cells A2:B10  var dataRange = sheet.getRange(startRow, 1, numRows, 2);  // Fetch values for each row in the Range.  var data = dataRange.getValues();  for (var i in data) {    var row = data[i];    var emailAddress = row[0]; // First column    var message = row[1]; // Second column    var subject = 'รบกวนพี่แคทตั้ง Auto Media สำหรับ Mini Program';    MailApp.sendEmail(emailAddress, subject, message);    }}}