Hello,
`File` and `Dbconn` point to resources handled by C libraries (`File` is actually C stdlib's `FILE*` for the C target), and as such should not be messed up with directly. For files, after closing, the C stdlib will see it doesn't point to an opened file and will set `errno` etc... (Nim checks this and raises an `IOError` afterwards). But if you set the pointer to `nil`, the C stdlib will try to dereference the pointer and the program will crash.
