Retrieve related data through id in text boxes ?

i want to retrieve data into text boxes through id or name. im using this code:

public partial class Form3 : Form

{

string connStr = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;

SqlCommand com;

//SqlDataAdapter sqlda;

//DataSet ds;

string str;

private void button4_Click(object sender, EventArgs e)

{

SqlConnection con = new SqlConnection();

con.Open();

str = "select * from Department where Code='" + codeTextBox.Text.Trim() + "'";

com = new SqlCommand(str, con);

SqlDataReader reader = com.ExecuteReader();

if (reader.Read())

{

dep_NameTextBox.Text = reader["Name"].ToString();

dep_TypeTextBox.Text = reader["Dep_Type"].ToString();

//TextBox4.Text = reader["ADDRESS"].ToString();

//TextBox5.Text = reader["PHNO"].ToString();

//TextBox6.Text = reader["STATE"].ToString();

reader.Close();

con.Close();

}

When i run the program it generate the error:

An unhandled exception of type 'System.NullReferenceException' occurred in voucher.exe

Additional information: Object reference not set to an instance of an object.

how to resolve it?

or how to retrieve related id data in text boxes in other query ?