Browsing during setup

[rerowep]

Describe the issue
There is a following error during setup after item indexing during browsing of brief view of documents:
Something bad happened; please try again later.

Following error is logged in server log:

  File "/Users/peter/devel/rero/rero-ils/rero_ils/modules/documents/serializers.py", line 76, in post_process_serialize_search
    metadata.get('pid')).is_available(viewcode)
AttributeError: 'NoneType' object has no attribute 'is_available'

To Reproduce
During setup after item indexing try to visit web address:
http://localhost:5000/global/search/documents?q=&page=1&size=10

Expected behavior
We can already browse documents without error.