Crawl fails with an error 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. )

Use the Object model to get all the SPGroup whose Description property is set to null and update the value with a string value.

Sample code which will fix the issue

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 using (SPSite oSite = new SPSite(textBox1.Text))
                {
                    using (SPWeb oWeb = oSite.OpenWeb())
                    {
                        SPGroupCollection oGroupColl = oWeb.Groups;
                        foreach (SPGroup oGroup in oGroupColl)
                        {
                            if (oGroup.Description == null)
                            {
                                oGroup.Description = "String";
                                oGroup.Update();
                            }
                        }
                    }
                }