Creating A ShoppingCart

[Gfaenza]

My problem is that everytime the DataRow tries to write in the DataTable a NullReferenceExeption occurs (Object reference not set to an instance of an object)

can anyone spot the problem

 DBConnection myConn = new DBConnection(); 
 
DateTime today = DateTime.Today;
int bottleID;
 
DataTable cart = new DataTable("MyShoppingCart");
 
DataColumn DC1;
DataColumn DC2;
DataColumn DC3;
DataColumn DC4;
 
protected void Page_Load(object sender, EventArgs e)
{
fillData();
 
if (Session["Cart"] == null)
{
CreateCart();
DataTable cart = (DataTable)Session["Cart"];
}
else
{
DataTable cart = (DataTable)Session["Cart"];
}
 
} 
 
 
public void CreateCart()
{
DC1 = new DataColumn("BottleID", System.Type.GetType("System.Int32"));
DC2 = new DataColumn("Quantity", System.Type.GetType("System.Int32"));
DC3 = new DataColumn("Price", System.Type.GetType("System.Double"));
DC4 = new DataColumn("Volume", System.Type.GetType("System.Int32"));
 
cart.Columns.Add(DC1);
cart.Columns.Add(DC2);
cart.Columns.Add(DC3);
cart.Columns.Add(DC4);
 
cart = (DataTable)Session["Cart"];
 
}
 
public void fillData()
{
lblClientID.Text = myConn.UserLoggedin(Session["Username"].ToString());
lblBottleID.Text = GetbottleInfo();
lblDate.Text = today.ToString();
 
} 
 
 
 
protected void btnAddToCart_Click(object sender, EventArgs e)
{
DataTable cart = (DataTable)Session["Cart"];
 
 
 
DataRow DR = cart.NewRow();
 
DR[DC1] = bottleID;
DR[DC2] = Convert.ToInt32(txtQty.Text);
DR[DC3] = getBottlePrice(bottleID);
DR[DC4] = cmbVol.SelectedValue;
 
cart.Rows.Add(DR); 
 
Response.Redirect("ShoppingCart.aspx");
}