'subtitle' missing from admin context on index page

[Zain Patel]

This looks like a bug introduced in Django 3.2 with the introduction of a new variable introduced in the base templates (django/contrib/admin/templates/base_site.html) named subtitle. This variable is passed in most places within the admin site, e.g in django/contrib/admin/options.py

Loading the admin index page with log-level debug shows the following exception/stack trace (that admittedly does not affect anything functional - simply clutters the logs):

Exception while resolving variable 'subtitle' in template 'admin/index.html'.
Traceback (most recent call last):
  File "...lib/python3.7/site-packages/django/template/base.py", line 829, in _resolve_lookup
    current = current[bit]
  File "...lib/python3.7/site-packages/django/template/context.py", line 83, in __getitem__
    raise KeyError(key)
KeyError: 'subtitle'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "...lib/python3.7/site-packages/django/template/base.py", line 835, in _resolve_lookup
    if isinstance(current, BaseContext) and getattr(type(current), bit):
AttributeError: type object 'RequestContext' has no attribute 'subtitle'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "...lib/python3.7/site-packages/django/template/base.py", line 843, in _resolve_lookup
    current = current[int(bit)]
ValueError: invalid literal for int() with base 10: 'subtitle'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "...lib/python3.7/site-packages/django/template/base.py", line 850, in _resolve_lookup
    (bit, current))  # missing attribute
django.template.base.VariableDoesNotExist: Failed lookup for key [subtitle] in

I believe this can be fixed by providing subtitle: None in the AdminSite.each_context` method to default it to that. I am happy to create a PR for this.

On second though - instead of hardcoding subtitle to None - potentially it should be a customisable option in the AdminSite itself, so:

class MySite(AdminSite):
  site_header = "blah"
  subtitle = "my subtitle"

?