Hi Ragi,
Thanks for the detailed error reporting, I've taken a look at the code, and I think I might know what's causing this.
Have you recently removed a section that contained an upload field? And if so, did you delete the schema directly from the schema directory?
It looks like that error could be thrown if CMS Builder finds an unlinked upload that belongs to a section that no longer exists. If I can confirm the issue, I'll make sure that we resolve it in our next release.
Cheers,
Greg
Greg Thomas
PHP Programmer - interactivetools.com