First check if the column is present or not and if in-case its present the column value has to be checked with DBNull.Value and not Null.
private void pcNext_Click(object sender, EventArgs e)
{
if (dt != null && dt.Rows.Count > 0 && dt.Rows.Contains("BillNo") && dt.Rows[0]["BillNo"] != DBNull.Value)
{
if (BILLNo == Convert.ToInt32(dt.Rows[0]["BillNo"].ToString()))
{
MessageBox.Show("Already at Last Page..!");
}
else
{
BILLNo++;
ShowData();
}
}
}