Sorry but I don't understand the question of "which stack". The error message says:

System.NullReferenceException: Object reference not set to an instance of object.

Then I get the Stack Trace as I already posted.

How do I determine what to look for to answer your question?

>Okay, this is a standard error message stack for web page load error, in which stack does the object null references gets returned?
>
>>>Normally on these error, you will get an error stack message as well, the function may have initiate in the Page_load by threw an error in lets said a business entity process function, so you should see the exception stack that will give you an indication where the error has occur. Here is an example of an error I just created to demo the error return structure.
>
>[NullReferenceException: Object reference not set to an instance of an object.]
> GoFish.ActiveRecord`1.GetColumnValue(String columnName) +108
> Commerce.Common.Order.get_CouponCodes() in App_Code\DataAccess\Store\Generated\Order.cs:358
> Checkout.SetOrderInfo() in d:\hshome\hspreps\sentimentalsolutions.com\Checkout.aspx.cs:170
> Checkout.Page_Load(Object sender, EventArgs e) in Checkout.aspx.cs:106
> System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +15
> System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +33
> System.Web.UI.Control.OnLoad(EventArgs e) +99
> System.Web.UI.Control.LoadRecursive() +47
>
>>>
>>>Lastly, you made ask the customer to check the application log file to see if the message was logged as well, that is pretty standard process today, logging all exceptions.
>>>
>>>Hope this helps.
>>
>>Unfortunately I have not implemented the application log file.
>>
>>The error does have the Stack Trace. I showed the top two lines of the Stack Trace in my initial message. The other lines of the Stack Trace don't seem to have any of my application pages or classes so I thought that they will not help me to find what causes the error.
>>
>>The other lines in the Stack trace are:
>>
>>

>>System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e)+15
>>System.Web.Util.CalliEventHandlerDelageProxy.Callback(Object sender, Eventargs e)+ 34
>>System.Web.UI.Control.OnLoad(EventArgs e)+ 99
>>System.Web.UI.Control.LoadRecursive()+47
>>System.Web.UI.Page.ProcessRequestMain(Boolen includeStagesBeforeAsyncPoint, Boolen includeStagesAfterAsycnPoint) +1062
>>

>>
>>Thank you.