未将对象引用设置为服务器上对象的实例。该对象与属性“ModifiedBy”关联

请检查下面的代码并建议我一个解决方案。错误说 内部：无法执行代码阶段，因为代码阶段引发异常：对象引用未设置为服务器上对象的实例。该对象与 ModifiedBy 属性关联。

//List list = ctx.Web.Lists.GetByTitle(Folder1[0]);
Microsoft.SharePoint.Client.Folder folder = ctx.Web.GetFolderByServerRelativeUrl(SPFolder);


//Load List , Root Folder and Files
//ctx.Load(list);
//ctx.Load(list.RootFolder);
//ctx.Load(list.RootFolder.Folders);
//ctx.Load(list.RootFolder.Files);
ctx.Load(folder);
//ctx.Load(folder.Files);
ctx.Load(folder.Files,File=>File.IncludeWithDefaultProperties(_file=>_file.ModifiedBy.Email));

//Execute Query
ctx.ExecuteQuery();
                
//Get Query Result in File Collection
//FileCollection fcol = list.RootFolder.Files;
FileCollection fcol = folder.Files;

//Declare Normal Collection
DataTable dt=new DataTable();
dt.Columns.Add("File Name",typeof(string));
//dt.Columns.Add("Checked Out By User",typeof(string));
//dt.Columns.Add("Check In Comment",typeof(string));
dt.Columns.Add("Modified By",typeof(string));
dt.Columns.Add("Last Modified Time",typeof(DateTime));
dt.Columns.Add("Created Time",typeof(string));


foreach(Microsoft.SharePoint.Client.File f in fcol)
{
                DataRow dr = dt.NewRow();
                dr["File Name"]=f.Name;
                //dr["Checked Out By User"]=f.CheckedOutByUser.ToString();
                //dr["Check In Comment"]=f.CheckInComment.ToString();
                dr["Modified By"]=f.ModifiedBy.Email.ToString();
                dr["Last Modified Time"]=Convert.ToDateTime(f.TimeLastModified);
                dr["Created Time"]=f.TimeCreated.ToString();
                dt.Rows.Add(dr);
}

OutputCollection = dt;