Hello,
This may be tricky to track down but I'll report it in case there is already a fix or if this information is helpful when combined with anyone else's report.
About 6 times now in the last 6 months, we've had a site (about 4 separate sites) with an issue accessing the Files section. We get this Object reference error with the files list.
In each time, there wasn't a known change on the site. Just our customer or someone working in the Files section. And, it's resolved with a recycle of the app pool. So, there is some temporary thing that occurs just sometimes.
Thanks,
Scott