Error in login form

Question

am trying to create a login form using sql and when the application runs, it crashes at

da.Fill(dt);  public partial class Form1 : Form
    {
        SqlConnection con = new SqlConnection();
        public Form1()
        {
            SqlConnection con = new SqlConnection();
            con.ConnectionString = @"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\Xavier\Documents\Data.mdf;Integrated Security=True";

            InitializeComponent();
        }
        private void pictureBox4_Click(object sender, EventArgs e)
        {
            SqlConnection con = new SqlConnection();
            con.ConnectionString = @"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\Xavier\Documents\Data.mdf;Integrated Security=True";
            con.Open();
            string Username = textBox1.Text;
            string Password = textBox2.Text;
            SqlCommand cmd = new SqlCommand("select userid,password from login where Username='" + textBox1.Text + "'and Password='" + textBox2.Text + "'", con);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            da.Fill(dt);
            if (dt.Rows.Count > 0)
            {
                this.Hide();
                Form2 f2 = new Form2();
                f2.Show();
            }
            else
            {
                MessageBox.Show("Invalid Login Credentials!");
            }
        }
    }
}