You might get a better response to this question over on the WebGrids forum.

It might also help you obtain a response if you can give more information about the problem.  An "object reference" error is very generic and could mean anything.  With this little information, people won't be able to help you.

Ensure that any Javascript files you have deployed (if you deploy them separately from the embedded resources in the assembly) match the version and build number of the WebGrid assembly you have deployed.  Any version or build mismatch here can cause you script errors.  Also make sure the browser's Temporary Internet Files cache is cleared (it will cache old versions of the script if you just updated) and that any IIS worker process in which your ASP.NET application runs has recycled (it will cache old versions of the assembly if you just updated).

You can then ask yourself some questions that ... while they are not a direct solution to your problem ... their answers will help you hone in on information that will help others help you with it.  Enable Script debugging in the browser.  What JavaScript file does this error message occur in?  do you get a line number in the error message?  what is it?  When you choose to Debug when you receive a prompt offering you a chance to debug, what is the line where the error occurs trying to do?  You might be able to tell from this whether the WebGrid is doing something with a row, column or band related to some feature you have enabled or some piece of data in your WebGrid.  If other forumgoers don't have the same features enabled as you or are not working with the same data, they perhaps won't see the same problem or be able to help, so that's why this information can be helpful.