[2.3] 500 error when uploading a new commissioning script

This bug is not related to #1712423 afaik as the steps to reproduce are dissimilar.

Steps to reproduce:

1. Go to Setting tab and click on 'Upload script' in the 'Commissioning scripts' section.
2. Click on 'Choose File' to upload a script file.
3. Pick the script file you would like to upload.
4. Click on the 'Upload' button.

500 stacktrace is produced:

2017-08-22 17:23:40 maasserver.utils.views: [error] 500 Internal Server Error @ /MAAS/commissioning-scripts/add/
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/django/core/handlers/base.py", line 132, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/utils/views.py", line 185, in view_atomic_with_post_commit_savepoint
    return view_atomic(*args, **kwargs)
  File "/usr/lib/python3.5/contextlib.py", line 30, in inner
    return func(*args, **kwds)
  File "/usr/lib/python3/dist-packages/django/contrib/auth/decorators.py", line 22, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/django/views/generic/base.py", line 71, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/django/views/generic/base.py", line 89, in dispatch
    return handler(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/django/views/generic/edit.py", line 249, in post
    return super(BaseCreateView, self).post(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/django/views/generic/edit.py", line 214, in post
    if form.is_valid():
  File "/usr/lib/python3/dist-packages/maasserver/forms/script.py", line 425, in is_valid
    if not self._form.is_valid():
  File "/usr/lib/python3/dist-packages/maasserver/forms/script.py", line 384, in is_valid
    self.instance.script.delete()
  File "/usr/lib/python3/dist-packages/django/db/models/base.py", line 891, in delete
    (self._meta.object_name, self._meta.pk.attname)
AssertionError: VersionedTextFile object can't be deleted because its id attribute is set to None.
2017-08-22 17:23:40 regiond: [info] ::1 POST /MAAS/commissioning-scripts/add/ HTTP/1.1 --> 500 INTERNAL_SERVER_ERROR (referrer: http://192.168.122.10/MAAS/commissioning-scripts/add/; agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.101 Safari/537.36)