Object not set to an instance of the object - on downloadlink from details view

Hi,
I have an issue on my details view download link.
I have been able to add two download links, to my details view. When I go to select either of the links I recieve an error.
Object reference not set to an instance of an object. I'm then told to "use the "new" keyword to create an insatnce of the object.
I'm not sure what this means or how I'm supposed to fix it.
I'm pretty sure the issue is becasue of the gridview data key name being the primary key of the table to establish the actual details view. Which makes me think that my download link code is incorrect.
<asp:GridView ID="gridProjects" runat="server" AutoGenerateColumns="False" AllowPaging="True" AllowSorting="True" DataKeyNames="projectID" DataSourceID="projectsDataSourceGrid" PageSize="8" CssClass="mGrid" EmptyDataText="No Applicable Data"> Any help would be greatly appr ...