Object reference not set to an instance of an object "when initializing value into session from child page to master page function

OrdrInfo ordinfo = (OrdrInfo)Session["addtocartlist"]; // OrdrInfo is class where i have arraylist.
shopCarInfor(ordinfo);

public void shopCarInfor(OrdrInfo order)
{

Session["addtocartlist"] = order;
}


//till now there is no problem. .

///child page:

protected void c1_row_command(object sender, C1GridViewCommandEventArgs e) // when this function calls using command name 
{
if (e.CommandName.ToLower() == "delete")
{
// i am deleting particulars and binding into class
OrdrInfo model = (OrdrInfo)Session["addtocartlist"];

}
//there is some needed that i need to update class value from child page to master pager 
//so, i called like below,
project.MasterPage.Site1 objsite = new MasterPage.Site1();

objsite.shopCarInfor(model);

}

//this objsite.shopCarInfor(model); calls correctly which is in master page also i am getting //value here

public void shopCarInfor(OrdrInfo order) // i am getting value in order
{

Session["addtocartlist"] = order; // when initializing order to session , i am getting error like "Object reference not set to an instance of an object".

}