### Apache Airflow version

main (development)

### What happened

Create a simple dag, allow it to completely run through. 

Next, when in grid view, on the left hand side click on the dag run at the top level. 

On the right hand side, then click on "Clear existing tasks". This will error with the following on the web server:

```
[2022-11-29 17:55:05,939] {app.py:1742} ERROR - Exception on /dagrun_clear [POST]
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 2525, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1822, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1820, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1796, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
  File "/opt/airflow/airflow/www/auth.py", line 47, in decorated
    return func(*args, **kwargs)
  File "/opt/airflow/airflow/www/decorators.py", line 83, in wrapper
    return f(*args, **kwargs)
  File "/opt/airflow/airflow/www/views.py", line 2184, in dagrun_clear
    confirmed=confirmed,
  File "/opt/airflow/airflow/www/views.py", line 2046, in _clear_dag_tis
    session=session,
  File "/opt/airflow/airflow/utils/session.py", line 72, in wrapper
    return func(*args, **kwargs)
  File "/opt/airflow/airflow/models/dag.py", line 2030, in clear
    exclude_task_ids=exclude_task_ids,
  File "/opt/airflow/airflow/models/dag.py", line 1619, in _get_task_instances
    tis = session.query(TaskInstance)
AttributeError: 'NoneType' object has no attribute 'query'
```
https://github.com/apache/airflow/blob/527fbce462429fc9836837378f801eed4e9d194f/airflow/models/dag.py#L1619

As per issue title, fails on main branch and `2.5.0rc2`. Works fine on `2.3.3` and `2.4.3`. 


### What you think should happen instead

Tasks within the dag should be cleared as expected. 

### How to reproduce

Run a dag, attempt to clear it within the UI at the top level of the dag. 

### Operating System

Ran via breeze

### Versions of Apache Airflow Providers

N/A

### Deployment

Other 3rd-party Helm chart

### Deployment details

Tested via breeze. 

### Anything else

Happens every time. 

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

RemovedInAirflow3Warning,
    TaskNotFound,
)
from airflow.models.abstractoperator import AbstractOperator
from airflow.models.base import Base, StringID
from airflow.models.dagcode import DagCode
from airflow.models.dagpickle import DagPickle
from airflow.models.dagrun import DagRun
from airflow.models.operator import Operator
from airflow.models.param import DagParam, ParamsDict
from airflow.models.taskinstance import Context, TaskInstance, TaskInstanceKey, clear_task_instances
from airflow.secrets.local_filesystem import LocalFilesystemBackend
from airflow.security import permissions
from airflow.stats import Stats
from airflow.timetables.base import DagRunInfo, DataInterval, TimeRestriction, Timetable
from airflow.timetables.interval import CronDataIntervalTimetable, DeltaDataIntervalTimetable
from airflow.timetables.simple import DatasetTriggeredTimetable, NullTimetable, OnceTimetable
from airflow.typing_compat import Literal
from airflow.utils import timezone
from airflow.utils.dag_cycle_tester import check_cycle
from airflow.utils.dates import cron_presets, date_range as utils_date_range
from airflow.utils.file import correct_maybe_zipped
from airflow.utils.helpers import at_most_one, exactly_one, validate_key
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.session import NEW_SESSION, provide_session
from airflow.utils.sqlalchemy import Interval, UtcDateTime, skip_locked, tuple_in_condition, with_row_locks
from airflow.utils.state import DagRunState, State, TaskInstanceState
from airflow.utils.types import NOTSET, ArgNotSet, DagRunType, EdgeInfoType
if TYPE_CHECKING:
    from types import ModuleType