When i am clicking once time in on button to view data in gridview, it works fine, but when i click again on button, it is giving me error.

Object reference not set to an instance of an object.

on below line 

 DataTable dt = (e.Row.DataItem as DataRowView).DataView.Table;

string currentCategory = "";
decimal wbSubTotal = 0;
decimal bSubTotal = 0;
decimal WBalanceTotal = 0;
decimal balanceTotal = 0;
int rowIndex = 0;
protected void GVBBSTK_RowCreated1(object sender, GridViewRowEventArgs e)
{
    wbSubTotal = 0;
    bSubTotal = 0;
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        DataTable dt = (e.Row.DataItem as DataRowView).DataView.Table;
        string category = dt.Rows[e.Row.RowIndex]["Supplier"].ToString();
        balanceTotal += Convert.ToDecimal(dt.Rows[e.Row.RowIndex]["Balance"]);
        WBalanceTotal += Convert.ToDecimal(dt.Rows[e.Row.RowIndex]["W_Balance"]);
        if (category != currentCategory)
        {
            if (e.Row.RowIndex > 0)
            {
                for (int i = rowIndex; i < e.Row.RowIndex; i++)
                {
                    wbSubTotal += Convert.ToDecimal(GVBBSTK.Rows[i].Cells[e.Row.Cells.Count - 1].Text);
                    bSubTotal += Convert.ToDecimal(GVBBSTK.Rows[i].Cells[e.Row.Cells.Count - 2].Text);
                }
                this.AddTotalRow("Sub Total", bSubTotal.ToString("N2"), wbSubTotal.ToString("N2"));
                rowIndex = e.Row.RowIndex;
            }
            currentCategory = category;
        }
    }
}