Q:

Cannot call method getRange of null Google Spreadsheet

I'm trying to post a form to Google Spreadsheet with the code below: 
function doGet(e){
  return handleResponse(e);
}

//  Enter sheet name where data is to be written below
    var SHEET_NAME = "orderSheet";

var SCRIPT_PROP = PropertiesService.getScriptProperties(); // new property service

function handleResponse(e) {
  var lock = LockService.getPublicLock();
  lock.waitLock(30000);  // wait 30 seconds before conceding defeat.

  try {

var doc = SpreadsheetApp.openById(SCRIPT_PROP.getProperty("key"));
var sheet = doc.getSheetByName(SHEET_NAME);

var headRow = e.parameter.header_row || 1;
var headers = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues()[0];
var nextRow = sheet.getLastRow()+1; // get next row
var row = []; 
// loop through the header columns
for (i in headers){
  if (headers[i] == "Timestamp"){ // special case if you include a 'Timestamp' column
    row.push(new Date());
  } else { // else use header name to get data
    row.push(e.parameter[headers[i]]);
  }
}

return ContentService
      .createTextOutput(JSON.stringify({"result":"success", "row": nextRow}))
      .setMimeType(ContentService.MimeType.JSON);
  } catch(e){

return ContentService
      .createTextOutput(JSON.stringify({"result":"error", "error": e}))
      .setMimeType(ContentService.MimeType.JSON);
  } finally { //release lock
lock.releaseLock();
  }
}

function setup() {
var doc = SpreadsheetApp.getActiveSpreadsheet();
SCRIPT_PROP.setProperty("key", doc.getId());
}

My spreadsheet: 
So as I see the problem is occurred because of spreadsheet name mismatch, but everything seems okay. SHEET_NAME is set to "orderSheet" so the name of spreadSheet. start() function ran successfully and app was deployed.
Full response that I'm getting: 
{
"result": "error",
"error": {
    "message": "Cannot call method \"getRange\" of null.",
    "name": "TypeError",
    "fileName": "macros",
    "lineNumber": 28,
    "stack": "\tat macros:28 (handleResponse)\n\tat macros:5 (doGet)\n"
    }
}

A:

It seems to me there is a confusion about "SHEET_NAME" : it's not the name of the spreadsheet like showed in the screen shot ! It has to be the name of the sheet in the bottom (like some "tab").

