Getting refs is not work properly with models.Window and aggregation

[Sergey Nesterenko]

I found out that getting refs in QuerySet doesn't work with models.Window when I try to use aggregate method. It happened when I tried to update Django to 4.2.7.

For example:

MyModel.objects.annotate(  
	new_ordering=Window(RowNumber(),  order_by=[F('ordering')])  
).aggregate(  
	wrong_count=Count('id', filter=Q(ordering=F('new_ordering')))
)

And as a result, it fails with:

Traceback (most recent call last):
  File "/main.py", line 38, in <module>
    result = ordering_query.aggregate(
  File "/python3.10/site-packages/django/db/models/query.py", line 592, in aggregate
    return self.query.chain().get_aggregation(self.db, kwargs)
  File "/python3.10/site-packages/django/db/models/sql/query.py", line 398, in get_aggregation
    aggregate = aggregate_expr.resolve_expression(
  File "/python3.10/site-packages/django/db/models/aggregates.py", line 70, in resolve_expression
    for ref in c.get_refs():
  File "/python3.10/site-packages/django/db/models/expressions.py", line 418, in get_refs
    refs |= expr.get_refs()
  File "/python3.10/site-packages/django/db/models/sql/where.py", line 233, in get_refs
    refs |= child.get_refs()
  File "/python3.10/site-packages/django/db/models/expressions.py", line 418, in get_refs
    refs |= expr.get_refs()
  File "/python3.10/site-packages/django/db/models/expressions.py", line 418, in get_refs
    refs |= expr.get_refs()
AttributeError: 'NoneType' object has no attribute 'get_refs'

It happens because of Window.get_source_expressions return:

def get_source_expressions(self):  
	return [self.source_expression, self.partition_by, self.order_by, self.frame]

and if we don't specify 'frame' (for example) it will be None in the list.

And in models.aggregates.Aggregate.resolve_expression when we try to find all refs ​https://github.com/django/django/blob/47f9b8dca16b1fdc054b338d81eb080eabad0edf/django/db/models/aggregates.py#L70C30-L70C30 it fails because of we have None in Window.get_source_expressions

def get_refs(self):  
	refs = set()  
	for expr in self.get_source_expressions():  
		refs |= expr.get_refs()  
	return refs