**Apache Airflow version**: 2.0.0, LocalExecutor

**Environment**: Docker on Win10 with WSL, official Python3.8 image

**What happened**:

When a DAG is running, and I delete task from the running DAG, the scheduler fails. When using Docker, upon automatic restart of the scheduler, the scheduler just fails again, perpetually.

![image](https://user-images.githubusercontent.com/46958547/103558919-a89f6e80-4eb5-11eb-8521-19b04ee3c690.png)

Note: I don't _know_ if the task itself was running at the time, but I would guess it was.

**What you expected to happen**:

The scheduler should understand that the task is not part of the DAG anymore and not fail.

**How to reproduce it**:

- Create a DAG with multiple tasks
- Let it run
- While running, delete one of the tasks from the source code
- See the scheduler break
airflow/models/dagrun.py
#
#
#
from datetime import datetime
from typing import TYPE_CHECKING, Any, Iterable, List, NamedTuple, Optional, Tuple, Union
from sqlalchemy import (
    Boolean,
    Column,
    DateTime,
    Index,
    Integer,
    PickleType,
    String,
    UniqueConstraint,
    and_,
    func,
    or_,
)
from sqlalchemy.exc import IntegrityError
from sqlalchemy.ext.declarative import declared_attr
from sqlalchemy.orm import backref, relationship, synonym
from sqlalchemy.orm.session import Session
from airflow import settings
from airflow.configuration import conf as airflow_conf
from airflow.exceptions import AirflowException