Very dangerous in an ASP.NET app to assume there will be data. A good way to quickly crash your website!
I can see two lines where the error may occur. Also, the first line has multiple possible opportunities for the error to occur.
As mentioned above, it is time to break out the debugger, roll up the sleeves, and do some sleuth work. If you are not familiar with debugging, then here is a great place to start: Basic Debugging with Visual Studio 2010 - YouTube