NEVER MIND. I found out that a different control on the page was generating an error which caused the overall problem.

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I have a simple Hierarchial Grid. It works exactly right when I have this for the datasource:
<asp:XmlDataSource ID="XmldsFlightReservedGrid" runat="server"datafile="C:\Work\FlightGrid.xml"
XPath="Flights/Flight">
asp:XmlDataSource>

However, if I try to store the file name and path in the web.config I get the following error:
If I try to open the page in a browser -
Server Error in '/' Application.
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Data source of type XmlDataSource is not supported.

And when I am in design view
Error Creating Control  - XmldsFlightReservedGrid
Property accessor 'DataFile' on object 'System.Web.UY.Design.WebControls.XmlDataSourceDesigner' threw the following exception:'Object reference not set to an instance of an object.'

Here are my settings. I am using NetAdvantage 11.1 and .NET framework 4.0
<asp:XmlDataSource ID="XmldsFlightReservedGrid" runat="server"datafile="<%$ AppSettings:XMLFlightReservedPath %>" XPath="Flights/Flight">
asp:XmlDataSource>

<appSettings>
    <add key="XMLFlightReservedPath" value="C:\Work\FlightGrid.xml"/>
appSettings>

Any help would be greatly appreciated.