**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 airflow.models.base import ID_LEN, Base
from airflow.models.taskinstance import TaskInstance as TI
from airflow.settings import task_instance_mutation_hook
from airflow.stats import Stats
from airflow.ti_deps.dep_context import DepContext
from airflow.ti_deps.dependencies_states import SCHEDULEABLE_STATES
from airflow.utils import callback_requests, timezone
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.session import provide_session
from airflow.utils.sqlalchemy import UtcDateTime, nulls_first, skip_locked, with_row_locks
from airflow.utils.state import State
from airflow.utils.types import DagRunType
if TYPE_CHECKING:
    from airflow.models.dag import DAG
class TISchedulingDecision(NamedTuple):
    """Type of return for DagRun.task_instance_scheduling_decisions"""
    tis: List[TI]
    schedulable_tis: List[TI]
    changed_tis: bool
    unfinished_tasks: List[TI]
    finished_tasks: List[TI]
class DagRun(Base, LoggingMixin):
    """
    DagRun describes an instance of a Dag. It can be created
    by the scheduler (for regular runs) or by an external trigger
    """
    __tablename__ = "dag_run"
    id = Column(Integer, primary_key=True)
    dag_id = Column(String(ID_LEN))
    execution_date = Column(UtcDateTime, default=timezone.utcnow)