Looking at the code for \assignfeedback_editpdf\combined_document, I'm seeing this line in refresh_files().
The $conversion variable is not defined so this is probably not doing what it's expected to. I don't see any conversion plugins that do anything in poll_conversion() but, if they did, I suspect they'd probably break if passed a null. Should it be $file instead of $conversion?
(I should note, this is not affecting me in any way and I don't know how to "reproduce" an error - just reporting it as I noticed it while looking at something else!)