There is already an open DataReader associated with this Connection which must be closed first error in c#

Question

MySqlCommand command = new MySqlCommand("select * from tbl_user where username = @uname", conn);
command.Parameters.AddWithValue("@uname", txtuname.Text);
conn.Open();

MySqlDataReader reader = command.ExecuteReader();

if (reader.HasRows)
{
    while (reader.Read())
    {
        MessageBox.Show("Username already used!");
    }
}
else
{
    string sql = "Insert into tbl_user(Username,Password,FirstName,LastName) values(@uname,@pword,@fname,@lname)";

    MySqlCommand sda = new MySqlCommand(sql, conn);
    sda.Parameters.AddWithValue("@uname", txtuname.Text);
    sda.Parameters.AddWithValue("@pword", txtpword.Text);

    sda.Parameters.AddWithValue("@fname", txtfname.Text);
    sda.Parameters.AddWithValue("@lname", txtlname.Text);

    sda.ExecuteNonQuery();

    MessageBox.Show("Successfully Signed Up!\n You can login now!");
    Form1 fm = new Form1();
    this.Hide();
    fm.ShowDialog();
}
reader.Close();