System.NullReferenceException: 未将对象引用设置到对象的实例！

using (SPWeb web = site.OpenWeb(webID))
                        {
                            SPList list = web.Lists[listID];
                            SPListItem item = list.GetItemById(itemID);                //取到想要设置的记录
                            if ("UserMulti" == field.TypeAsString)                     //判断该字段是否设置了“允许多重选择”
                            {
                                SPFieldUserValueCollection members = (SPFieldUserValueCollection)item[field.Title];    //取出所有数据内容
                                foreach (SPFieldUserValue member in members)
                                {
                                    if (member.User != null)                            //判断该对象是否是用户类型
                                    {
                                        principal.AddUser(member.User);                 //添加用户
                                    }
                                    else
                                    {
                                        try 
 
                                        {
                                            SPGroup group = web.SiteGroups[member.LookupValue];         //获取用户组
                                            principal.AddGroup(group);                                  //添加用户组
                                        }
                                        catch
                                        {
                                            continue;
                                        }
                                    }
                                }
                            }

在foreach | 这里报错：System.NullReferenceException: 未将对象引用设置到对象的实例。
[解决办法]
SPFieldUserValueCollection members = new SPFieldUserValueCollection(web,field.InternalName);