[sobjects/notification] fix broken email notifications
In https://hg.logilab.org/review/cubicweb/rev/8caa109dfe945592a34904bb56971cc57d1c6084 EntityView.w become a method using EntityView._w
In notifications we reset "self.w" for every notifications since we handle multiple streams.
We should set "self._w" to None instead, otherwise subsequent calls to "self.w()" will fail with TypeError: 'NoneType' object is not callable.
addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile