How to clear Error in asp.net

protected void btnAdd_Click(object sender, EventArgs e)
    {
        int count = 0;

        DataTable myTable = new DataTable();
        myTable = CreateDataTable();
        if (Convert.ToInt32(Session["Count"]) != 0)
            myTable = (DataTable)Session["myDatatable"];

        // tot = Convert.ToInt32(tot) + 12wConvert.ToInt32(txtAmt.Text);

        DataRow row;
        row = myTable.NewRow();
        count = myTable.Rows.Count + 1;

        row["Size"] = cmbSize.SelectedItem.Text;
        row["SizeID"] = cmbSize.SelectedValue;

        row["Rate"] = Convert.ToInt64(txtRate.Text);
        row["Noofcopies"] = txtNoofcopies.Text;
        row["Total"] = txtTotal.Text;
        myTable.Rows.Add(row);
        Session["Count"] = Convert.ToInt32(Session["Count"]) + count;
        Session["myDatatable"] = myTable;
        //lblTotal.Text = Convert.ToString(tot);
        Grid();
    }