Error Occurs at the time of assigning data to DataSource

Question

Below code works the first time - at Page_Load it's working, data showing in the gridview control. But when I add new data to the database and retrieve I get an error.

I put the breakpoint and I checked step-by-step: DataTable dtsent is not null, it has data, but when it comes to gridSent.DataSource=dtsent an error occurs:

NullReferenceException was unhandled by user code
Object reference not set to an instance of an object

public void BindGridSent()
    {
        try
        {
            BusinessObject.BusinessObject bogetMS = new BusinessObject.BusinessObject();
            bogetMS.UserID = Convert.ToInt32(HttpContext.Current.Session["uid"]);

            DataTable dtsent = new DataTable();
            dtsent.Columns.Add("msgID");
            dtsent.Columns.Add("ToUsername");
            dtsent.Columns.Add("msgContent");
            dtsent.Columns.Add("msg_sent_date");
            dtsent.Columns.Add("fullmsgContent");
            DataRow dwsent = null;
            DataSet dssent = businssLogicMS.MessageSentList_BAL(bogetMS);


            if (dssent.Tables[0].Rows.Count > 0)
            {
                for (int i = 0; i < dssent.Tables[0].Rows.Count; i++)
                {
                    dwsent = dtsent.NewRow();
                    string msgID = dssent.Tables[0].Rows[i]["msgID"].ToString();
                    dwsent["msgID"] = msgID;
                    //string username="&nbsp;&nbsp;<b>" + dssent.Tables[0].Rows[i]["ToUsername"].ToString() + "</b>&nbsp;&nbsp;&nbsp;";
                    string username = dssent.Tables[0].Rows[i]["ToUsername"].ToString();
                    dwsent["ToUsername"] = username;
                    string stmsg = dssent.Tables[0].Rows[i]["msgContent"].ToString();
                    string message = stmsg.Substring(0, Math.Min(stmsg.Length, 80)) + "...";
                    string editmsgbody = message.ToString().Replace("<br />", Environment.NewLine);
                    dwsent["msgContent"] = editmsgbody;
                    dwsent["fullmsgContent"] = stmsg;
                    string date = String.Format("{0:MMM dd}", dssent.Tables[0].Rows[i]["msg_sent_date"]);
                    dwsent["msg_sent_date"] = "<font color='Red'>" + date + "</font>";
                    dtsent.Rows.Add(dwsent);
                }

                gridSent.DataSource = dtsent; --- Error Occurs Here
                gridSent.DataBind();
            }
            else
            {
                dtsent.Rows.Add(dtsent.NewRow());
                gridSent.DataSource = dtsent;
                gridSent.DataBind();

                int columncount = gridSent.Rows[0].Cells.Count;
                gridSent.Rows[0].Cells.Clear();
                gridSent.Rows[0].Cells.Add(new TableCell());
                gridSent.Rows[0].Cells[0].ColumnSpan = columncount;
                gridSent.Rows[0].Cells[0].Text = "No Record Found....";
            }
        }
        catch (Exception ex)
        {
            throw ex;
        }
    }