I got Error Mssage How to resolve thismessage

protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)
    {
        GridView _gridView = (GridView)sender;

        if (e.RowIndex > -1)
        {
            // Loop though the columns to find a cell in edit mode
            for (int i = _firstEditCellIndex; i < _gridView.Columns.Count; i++)
            {
                // Get the editing control for the cell
                Control _editControl = _gridView.Rows[e.RowIndex].Cells[i].Controls[0];
                if (_editControl.Visible)
                {
                    int _dataTableColumnIndex = i - 1;

                    try
                    {
                        // Get the id of the row
                        Label idLabel = (Label)_gridView.Rows[e.RowIndex].FindControl("lblMessage");
                        String QuestionNo = (lblMessage.Text).ToString();
                        // Get the value of the edit control and update the DataTable
                        DataTable dt = _sampleData;
                        DataRow dr = dt.Rows.Find(QuestionNo);
                        dr.BeginEdit();
                        if (_editControl is TextBox)
                        {
                            dr[_dataTableColumnIndex] = ((TextBox)_editControl).Text;
                        }
                        //else if (_editControl is DropDownList)
                        //{
                        //    dr[_dataTableColumnIndex] = ((DropDownList)_editControl).SelectedValue;
                        //}
                        //else if (_editControl is CheckBox)
                        //{
                        //    dr[_dataTableColumnIndex] = ((CheckBox)_editControl).Checked;
                        //}
                        dr.EndEdit();

                        // Save the updated DataTable
                        _sampleData = dt;

                        // Clear the selected index to prevent 
                        // another update on the next postback
                        _gridView.SelectedIndex = -1;

                        // Repopulate the GridView
                        _gridView.DataSource = dt;
                        _gridView.DataBind();
                    }
                    catch (ArgumentException)
                    {
                        this.lblMessage.Text += "Error updating GridView row at index " + e.RowIndex + "<br />";

                        // Repopulate the GridView
                        _gridView.DataSource = _sampleData;
                        _gridView.DataBind();
                    }
                }
            }
        }
    }