EPUB Thumbnail generation gives url is undefined error

[rtibbles]

    @rtibbles this has fixed the thumbnail generation issue for all resources except for the EPUB:

Originally posted by @pcenov in #3768 (comment)

[rtibbles]

@pcenov I was unable to replicate this either locally or on hotfixes. Which specific EPub is generating this error?

[pcenov]

Hi @rtibbles after further investigation this issue is valid only for EPUB files downloaded from Google's library and is not happening for the EPUBs in the QA Channel. I've had no issues with these before so it didn't occur to me that they might be corrupted or different in any way from the rest. Here's one of those:

TestGoogleEPUB.zip

[rtibbles]

Great, I've replicated with this file!

[rtibbles]

So, the issue here is that we are trying to tell the user that we can't generate a thumbnail for this EPUB, but then are still trying to make a thumbnail, which is giving the error. My fix won't allow generation of thumbnails for this EPUB, but will prevent the console error from occurring.

[rtibbles]

Fixed in #3780