Raise ImproperlyConfigured when Feed.link is missing instead of failing with AttributeError

[yureiblack]

Currently, the Feed.get_feed() method retrieves the feed's link using:

link = self._get_dynamic_attr("link", obj)

If a Feed subclass does not define a link attribute, link becomes None.
This results in an unhelpful exception when Django later calls:

link = add_domain(current_site.domain, link, request.is_secure())

Since add_domain() calls url.startswith(), a missing link
causes:

AttributeError: 'NoneType' object has no attribute 'startswith'

This error does not clearly indicate the actual problem.

According to Django documentation, every Feed class must define
link. Therefore, it should raise a more appropriate and explicit
exception.

The attached PR adds the following check:

if link is None:

raise ImproperlyConfigured(

"Feed class must define a 'link' attribute."

)

This results in a clearer error message and avoids unexpected crashes.

PR: ​https://github.com/django/django/pull/20371