how to solve "Object reference not set to an instance of an object."

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();
            }
            }
        }

Before use a field from reader please check that field should not be null

if (!Convert.IsDBNull(oReader["FEEDBACK_DATE"]))
{
     //your code
}

try this one...

private void pcNext_Click(object sender, EventArgs e)
{
    if(dt.Rows[0]["BillNo"] != null){
        if (BILLNo == Convert.ToInt32(dt.Rows[0]["BillNo"].ToString()))
        {
            MessageBox.Show("Already at Last Page..!");
        }
        else
        {
            BILLNo++;
            ShowData();
        }
    }
}

Debug and step through your source code.
Find the object that is null.

hello friend i thing don't convert .tostring...then it execute

just needed to keep the "BILLNo" in separate variable(bill) and then call that again there.. like,

private void pcNext_Click(object sender, EventArgs e)
{
if (BILLNo ==bill)
{
MessageBox.Show("Already at Last Page..!");
}
else
{
BILLNo++;
ShowData();
}
}

just needed to keep the "BILLNo" in separate variable(bill) and then call that again there.. like,

private void pcNext_Click(object sender, EventArgs e)
{
if (BILLNo ==bill)
{
MessageBox.Show("Already at Last Page..!");
}
else
{
BILLNo++;
ShowData();
}
}