KeyError exception when required metadata is missing

I'm using mayan v4.0.4. Trying to add a document where a required metadata is missing leads to a KeyError exception.

To reproduce:

• Create metadata type and document type using the metadata as "required".
• Create new document but leave the required metadata empty

Expected behavior: The form should remind the user to fill in the required metadata.

Actual behavior:

Server error

There's been an unexpected error.

In the logs:

Traceback (most recent call last):
  File "/opt/mayan-edms/lib/python3.7/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/opt/mayan-edms/lib/python3.7/site-packages/django/views/generic/base.py", line 71, in view
    return self.dispatch(request, *args, **kwargs)
  File "/opt/mayan-edms/lib/python3.7/site-packages/mayan/apps/sources/wizards.py", line 59, in dispatch
    return super().dispatch(request, *args, **kwargs)
  File "/opt/mayan-edms/lib/python3.7/site-packages/formtools/wizard/views.py", line 244, in dispatch
    response = super().dispatch(request, *args, **kwargs)
  File "/opt/mayan-edms/lib/python3.7/site-packages/django/views/generic/base.py", line 97, in dispatch
    return handler(request, *args, **kwargs)
  File "/opt/mayan-edms/lib/python3.7/site-packages/formtools/wizard/views.py", line 294, in post
    if form.is_valid():
  File "/opt/mayan-edms/lib/python3.7/site-packages/django/forms/formsets.py", line 301, in is_valid
    self.errors
  File "/opt/mayan-edms/lib/python3.7/site-packages/django/forms/formsets.py", line 281, in errors
    self.full_clean()
  File "/opt/mayan-edms/lib/python3.7/site-packages/django/forms/formsets.py", line 329, in full_clean
    form_errors = form.errors
  File "/opt/mayan-edms/lib/python3.7/site-packages/django/forms/forms.py", line 180, in errors
    self.full_clean()
  File "/opt/mayan-edms/lib/python3.7/site-packages/django/forms/forms.py", line 382, in full_clean
    self._clean_form()
  File "/opt/mayan-edms/lib/python3.7/site-packages/django/forms/forms.py", line 409, in _clean_form
    cleaned_data = self.clean()
  File "/opt/mayan-edms/lib/python3.7/site-packages/mayan/apps/metadata/forms.py", line 104, in clean
    if required and not self.cleaned_data['value'] and not self.cleaned_data.get('update'):
KeyError: 'value'