protect against 3rd parties creating NULL children

thanks for the module discrimination

how are you using itweak_upload?
with core's upload, comment_upload, filefield perhaps?

I mean, the attachment is related to comment_form or node_form?
(note that both are involved when comment_driven gets in)

first of all, don't panic, this "nasty" assertion is not addressing compatibility issues
it's the lack of test which made me guard myself against new code braking working functions

if this assertions shows up, most likely there is another module doing something wrong (in this case itweak_upload, as you accurately identified)
(otherwise I got blind and broke my own code)

you can find a solution to your problem here #737044: avoid creating a NULL form child $form['attachments']['wrapper']['files']

in addition, our own protection against 3rd party modules doing silliness was also committed to HEAD
so you won't need to patch itweak_upload

thanks to you for your accurate report

empty children are not a problem as long as they are empty arrays
the problem was introduced by NULL children as a matter of fact

should no longer apply for unstable5