Object reference not set to an instance of an object.

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C#

ASP.NET

Dear All,

I am new to Programming,

I Am doing the 3 tier Architecture Example.

Here I am Getting The Following Error..
Object reference not set to an instance of an object.

The Code is As following

Default Aspx.cs:

public partial class Default2 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void BtnAdd_Click(object sender, EventArgs e)
    {
        CustomerBAL bal = new CustomerBAL();
        try
        {

            if (bal.Insert(TxtCustomerID.Text,TxtFirstName.Text,TxtLastName.Text) > 0)
            {
                LblResult.Text = "Record Inserted Sucessfully";
            }
            else
            {
                LblResult.Text = "Records Not Inserted Sucessfully";
            }
        }
        catch (Exception ex)
        {
            LblResult.Text = ex.Message;
        }
        finally
        {
            bal = null;
        }
    }
}

CustomerBAL.aspx.cs

public class CustomerBAL
{
    public CustomerBAL()
    {

    }
    public int Insert(string CustomerID,String FirstName, string LastName)
    {
        CustomerDAL Dal= new CustomerDAL();
        try
        {
            return Dal.Insert(CustomerID,FirstName,LastName);

        }
        catch
        {
            throw;

        }
        finally
        {
            Dal= null;

        }
    }
}

CustomerDAL.aspx.cs

public class CustomerDAL
{
    public CustomerDAL()
    {

    }
    public int Insert(String CustomerID, String FirstName, String LastName)
    {
        SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["Stcon"].ConnectionString);
        string str1 = ("Insert Into Architechture_tier(CustomerID,FirstName,LastName) values (@CustomerID,@FirstName,@LastName)");
        //str1= str1+ "values(@CustomerID,@FirstName,@LastName)";
        SqlCommand cmd = new SqlCommand(str1, con);
        cmd.CommandType = CommandType.Text;
        try
        {
            cmd.Parameters.Add(new SqlParameter("@CustomerID", SqlDbType.VarChar));
            cmd.Parameters["@CustomerID"].Direction = ParameterDirection.Input;
            cmd.Parameters["@CustomerID"].Size = 20;
            cmd.Parameters["@CustomerID"].Value = CustomerID;

            cmd.Parameters.Add(new SqlParameter("@FirstName", SqlDbType.VarChar));
            cmd.Parameters["@FirstName"].Direction = ParameterDirection.Input;
            cmd.Parameters["@FirstName"].Size = 25;
            cmd.Parameters["@FirstName"].Value = FirstName;

            cmd.Parameters.Add(new SqlParameter("@LastName", SqlDbType.VarChar));
            cmd.Parameters["@LastName"].Direction = ParameterDirection.Input;
            cmd.Parameters["@LastName"].Size = 25;
            cmd.Parameters["@LastName"].Value = LastName;
            con.Open();
            return cmd.ExecuteNonQuery();
        }
        catch
        {
            throw;
        }
        finally
        {
            con.Close();
        }
    }
}

please Help me with The same...