While binding the gridview throwing an error in a process?

NpgsqlDataAdapter adp;
    NpgsqlConnection conn = new NpgsqlConnection(ConfigurationManager.ConnectionStrings["projop"].ConnectionString);
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            BindPrimaryGrid();
            BindSecondaryGrid();
        }
    }

     private void BindSecondaryGrid()
    {
        DataTable dt = (DataTable)ViewState["SelectedRecords"];
        gvSelected.DataSource = dt;
        gvSelected.DataBind();
    }
    private void BindPrimaryGrid()
    {
        DataTable dt = new DataTable();
        adp = new NpgsqlDataAdapter("select user_id, username,screen_name from users", conn);
        dt = new DataTable("users");
        adp.Fill(dt);
        gvAll.DataSource = dt;
        gvAll.DataBind();

protected void btnSubmit_Click(object sender, EventArgs e)
    {
        GetData();
        SetData();
        BindSecondaryGrid();
    }
private void GetData()
    {
        DataTable dt;
        if (ViewState["SelectedRecords"] != null)
            dt = (DataTable)ViewState["SelectedRecords"];
        else
            dt = CreateDataTable();
        CheckBox chkAll = (CheckBox)gvAll.HeaderRow
                            .Cells[0].FindControl("chkAll");
        for (int i = 0; i < gvAll.Rows.Count; i++)
        {
            if (chkAll.Checked)
            {
                dt = AddRow(gvAll.Rows[i], dt);
            }
            else
            {
                CheckBox chk = (CheckBox)gvAll.Rows[i]
                                .Cells[0].FindControl("chk");
                if (chk.Checked)
                {
                    dt = AddRow(gvAll.Rows[i], dt);
                }
                else
                {
                    dt = RemoveRow(gvAll.Rows[i], dt);
                }
            }
        }
        ViewState["SelectedRecords"] = dt;
    }

    private void SetData()
    {
        CheckBox chkAll = (CheckBox)gvAll.HeaderRow.Cells[0].FindControl("chkAll");
        chkAll.Checked = true;
        if (ViewState["SelectedRecords"] != null)
        {
            DataTable dt = (DataTable)ViewState["SelectedRecords"];
            for (int i = 0; i < gvAll.Rows.Count; i++)
            {
                CheckBox chk = (CheckBox)gvAll.Rows[i].Cells[0].FindControl("chk");
                if (chk != null)
                {
                    DataRow[] dr = dt.Select("CustomerID = '" + gvAll.Rows[i].Cells[1].Text + "'");
                    chk.Checked = dr.Length > 0;
                    if (!chk.Checked)
                    {
                        chkAll.Checked = false;
                    }
                }
            }
        }
    }

private DataTable AddRow(GridViewRow gvRow, DataTable dt)
    {
        DataRow[] dr = dt.Select("CustomerID = '" + gvRow.Cells[1].Text + "'");
        if (dr.Length <= 0)
        {
            dt.Rows.Add();
            dt.Rows[dt.Rows.Count - 1]["CustomerID"] = gvRow.Cells[1].Text;
            dt.Rows[dt.Rows.Count - 1]["ContactName"] = gvRow.Cells[2].Text;
            dt.Rows[dt.Rows.Count - 1]["Complete Name"] = gvRow.Cells[3].Text;
            dt.AcceptChanges();
        }
        return dt;
    }

    private DataTable RemoveRow(GridViewRow gvRow, DataTable dt)
    {
        DataRow[] dr = dt.Select("CustomerID = '" + gvRow.Cells[1].Text + "'");
        if (dr.Length > 0)
        {
            dt.Rows.Remove(dr[0]);
            dt.AcceptChanges();
        }
        return dt;
    }