I was trying to attach a image from the files that has been already uploaded previously, and the iframe just won't load, got the following error:

ERROR [User Error]: Bad RecordClassName '' and $baseClass not set
IN GET /admin/EditForm/field/Thumbnail/EditFileForm/field/ExistingFile/tree?forceValue=&SecurityID=a3b5fd2f55e54f97db5b000b2949f2a883f3292f

Just wondering if this is a bug from ss2.4.5

Just did a test on ss2.4.1, attach image this way is fine.

I have attached a screen shot of the error message.

I had a more close look. I used to have following custom image class:
'RegionPageThumbnail_CustomImage', 'InfrastructurePageThumbnail_CustomImage', 'AboutUsPageThumbnail_CustomImage'

Since they all share the same characteristics, I created a 'DefaultThumbnail_CustomImage,' and change the code to reference this default image class instead of individual separate class, and REMOVED these three image class. Now after dev/rebuild, I had a look at the 'File' table in the database, those images created previously, while they were referencing old image class has a empty ClassName, which makes sense because these image class no longer exist. Would this be the cause of 'BadRecordClassName'?

The ClassName in File table is a enum field, which means they must use the value included in this enum(), how could it be empty???

Does this mean, if you had a image class before and had upload some image using this class, you can not remove this class, because these images will be left without an associated image class?

I just removed those images in the File table that is without an associated image class. The the ajax iframe works.

This just confirm there is bug.