gridview postback not being posted - VS2008 - asp.net

Question

This is my aspx:
<asp:UpdatePanel ID="resultPanel" runat="server" UpdateMode="Conditional">
<Triggers>
<asp:AsyncPostBackTrigger ControlID="AddDocument" />
</Triggers>
<ContentTemplate>
<asp:GridView ID="gridView" runat="server" AutoGenerateColumns="False" EnableSortingAndPagingCallbacks="True"
AllowPaging="True" DataSourceID="FilesObjectDataSource" PageSize="5" OnRowCommand="gridView_RowCommand"
DataKeyNames="FileGuid" HorizontalAlign="Left" Width="100%" BorderStyle="Solid"
BorderColor="Black">
<Columns>
<asp:BoundField DataField="RID" HeaderText="ID" ReadOnly="True"></asp:BoundField>
<asp:BoundField DataField="Category" HeaderText="SubCategory" ReadOnly="True">
</asp:BoundField>
<asp:BoundField DataField="FileTypeName" HeaderText="Type" ReadOnly="True">
</asp:BoundField>
<asp:BoundField DataField="FileGUID" Visible="false" />
<asp:ButtonField Text="X" ButtonType="Button" ItemStyle-Width="20px" CommandName="DelFile">
<ItemStyle Width="20px" />
</asp:ButtonField>
</Columns>
<RowStyle CssClass="RowStyle" />
<EmptyDataRowStyle CssClass="EmptyRowStyle" />
<PagerStyle CssClass="PagerStyle" />
<SelectedRowStyle CssClass="SelectedRowStyle" />
<HeaderStyle CssClass="HeaderStyle" />
<EditRowStyle CssClass="EditRowStyle" />
<AlternatingRowStyle CssClass="AltRowStyle" />
</asp:GridView>
</ContentTemplate>
This is my code behind for the ButtonField marked 'X'
protected void gridView_RowCommand(Object sender, GridViewCommandEventArgs e)
{
if (e.CommandName == "DelFile")
{
//Selected Row
int rowIndex = Convert.ToInt32(e.CommandArgument);
fileGuid = new Guid(gridView.DataKeys[rowIndex].Values["FileGuid"].ToString());
}
}
Sometimes itt results in object ref not set and at times it works. Not sure why this happens in production and not in dev and testing.

Answer 1

I'm not sure if it makes any difference, but but the case in ("FileGuid")
DataKeyNames="FileGuid"
and
fileGuid = new Guid(gridView.DataKeys[rowIndex].Values["FileGuid"].ToString());
doesn't match ("FileGUID")
<asp:BoundField DataField="FileGUID" Visible="false" />

Answer 2

You can use like this for accessing value at row command event
protected void gridView_RowCommand(object sender, GridViewCommandEventArgs e)
{
if (e.CommandName == "DelFile")
{
int index = Convert.ToInt32(e.CommandArgument);
int documentID = Convert.ToInt32(gridView.DataKeys[index].Value);
// Write your further code
}
}