Lỗi khi đổ dữ liệu vào datagridview (Fill: SelectCommand.Connection property has not been initialized.)

Mình mới học về C#, khi chạy thì bị lỗi như này <Fill: SelectCommand.Connection property has not been initialized.>
Các bạn chỉ rõ cụ thể cho mình với.

Đây là class functions:

> using System;
> using System.Collections.Generic;
> using System.Linq;
> using System.Text;
> using System.Data;
> using System.Data.SqlClient;
> using System.Windows.Forms;

> namespace test1
> {
>     class Functions
>     {
>         public static SqlConnection Conn;

>              public static void Connect()
>              {
>                  Conn = new SqlConnection();
>                  Conn.ConnectionString = "Data Source=.\SQLExpress;AttachDbFilename=C:\Users\Fat Nhi\Documents\Visual Studio 2010\Projects\test1\test1\Database\test1.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True";
>                  try
>                  {
>                      Conn.Open();
>                  }
>                  catch (Exception ex)
>                  {
>                      MessageBox.Show(ex.ToString());
>                  }
>              } 

>         public static DataTable GetDataToTable(string sql)
>         {
>             DataTable table = new DataTable();
>             SqlDataAdapter mydata = new SqlDataAdapter(sql, Conn);
>             mydata.Fill(table);
>             return table;
>         }

>         public static void Disconnect()
>         {
>             if (Conn.State == ConnectionState.Open)
>             {
>                 Conn.Close();
>                 Conn.Dispose();
>                 Conn = null;
>             }
>         }
> }

Đây là form chính

> using System;
> using System.Collections.Generic;
> using System.ComponentModel;
> using System.Data;
> using System.Drawing;
> using System.Linq;
> using System.Text;
> using System.Windows.Forms;
> using System.Data.SqlClient;

> namespace test1
> {
>     public partial class Form1 : Form
>     {
>         public Form1()
>         {
>             InitializeComponent();
>         }

>         private void Form1_Load(object sender, EventArgs e)
>         {
>             btnLuu.Enabled = false;
>             btnBoqua.Enabled = false;
>             txtMahang.Enabled = false;
>             LoadDataGridView();
>         }

>         DataTable tbl;

>         private void LoadDataGridView()
>         {
>             string sql;
>             sql = "select * from tblHanghoa";
>             tbl = Functions.GetDataToTable(sql);
>             dgv.DataSource = tbl;
>          }
> }