Data is Null

The SharePoint item being crawled returned an error when requesting data from the web service. ( Error from SharePoint site: Data is Null. This method or property cannot be called on Null values. )

I found the code:
web.SiteGroups.Add(groupName, owner, null, null);

Solution:

It seems that the "default user" parameter is not the culprit here after all, but the fourth parameter is the one that actually causes this error. This is a description parameter, and even an empty string is OK. See this Microsoft Support article http://support.microsoft.com/kb/2323206