Problem with ModalPopupExtender(AJAX) + C#

[uicouic]

Hi all.
I am having trouble getting a ModalPopupExtender to display with the corresponding data. This will be triggered by clicking on a "Details" button in a gridview. Eg. by clicking on Row1 details button, ModalPopup will display Row1 information

I suspect this might be due to the following section:

protected void grdBookings_RowDataBound(object sender, GridViewRowEventArgs e)
{
 if (e.Row.RowType == DataControlRowType.DataRow)
            {
                Button Details1 = (Button)e.Row.FindControl("details1");
                Details1.Attributes.Add("onclick", "return ShowMyModalPopup('" + grdBookings.DataKeys[e.Row.RowIndex].Values + "')");
            }
}
 

ModalPopupExtender Code:

<asp:ModalPopupExtender ID="ModalPopupExtender1" BehaviorID="ModalPopupExtender1Behaviour" runat="server" 
PopupControlID="Panel1" TargetControlID="details1" BackgroundCssClass="modalBackground" 
OnCancelScript="HideModalPopup()" CancelControlID="Close"></asp:ModalPopupExtender>
 

There is a warning stating Possible NullReferenceException at line grdBookings.DataKeys[e.Row.RowIndex]. I have not stated the DataSource in the gridview itself as Im not sure how to do so since Im using a custom Stored Procedure. Could this be a cause?

I hope those in the know could help me as much as possible, thank you.