Problem with my save dialog logic

Question

Hi,

 

The code that you see its help to remove (ou delete) one column. Its works fine.

 

But the problem is I need to put a condition save logic. Its should show a dialog that allow if or not possible to save. Its show this dialog message but when I click yes its display this message error: Additional information: object reference not defined for instance object

private void btnDelete_Click(object sender, EventArgs e)
{
if (conn.State != ConnectionState.Open)
conn.Open();
OleDbCommand cmd = new OleDbCommand();
comm.Connection = conn;
if (MessageBox.Show("Are you sure to save?", "Save window", MessageBoxButtons.YesNo) == DialogResult.Yes)
{
try
{
cmd = new OleDbCommand("ALTER TABLE Billing DROP COLUMN ID ", conn);
comm.ExecuteNonQuery();
}
Finally
{
conn.Close();
}
}
}