C# - Using A Popup Window To Display Extra Information?

In my code I am using a popup window to display extra information. I also have the ability to export the information in the main window to Excel.The problem is, after the window pops up -> I see the info -> I close the popup window -> but if I try the export to Excel button, it throws the exception "null object referrence" (if I use a try/catch, the exception doesn't occur - but I don't get any information). In the export function I am doing something like this:

{
//some code .... here
con.close();
session["dss"] = mydataset;
}

In the export button click event:

system.data.dataset dss = (system.data.dataset)session["dss"];
//then some work on this

I think, probably when the popup window opens it ends the execution and that's why when I come back to the main window and try the export button the values for the tables and all goes out of scope.Also, if I refresh the main page after closing the popup window I don't have any issue and can export the data.