**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
external_trigger: Optional[bool] = None,
        no_backfills: bool = False,
        run_type: Optional[DagRunType] = None,
        session: Session = None,
        execution_start_date: Optional[datetime] = None,
        execution_end_date: Optional[datetime] = None,
    ) -> List["DagRun"]:
        """
        Returns a set of dag runs for the given search criteria.
        :param dag_id: the dag_id or list of dag_id to find dag runs for
        :type dag_id: str or list[str]
        :param run_id: defines the run id for this dag run
        :type run_id: str
        :param run_type: type of DagRun
        :type run_type: airflow.utils.types.DagRunType
        :param execution_date: the execution date
        :type execution_date: datetime.datetime or list[datetime.datetime]
        :param state: the state of the dag run
        :type state: str
        :param external_trigger: whether this dag run is externally triggered
        :type external_trigger: bool
        :param no_backfills: return no backfills (True), return all (False).
            Defaults to False
        :type no_backfills: bool
        :param session: database session
        :type session: sqlalchemy.orm.session.Session
        :param execution_start_date: dag run that was executed from this date
        :type execution_start_date: datetime.datetime
        :param execution_end_date: dag run that was executed until this date
        :type execution_end_date: datetime.datetime