**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
"""
        Get a single DAG Run
        :param session: Sqlalchemy ORM Session
        :type session: Session
        :param dag_id: DAG ID
        :type dag_id: unicode
        :param execution_date: execution date
        :type execution_date: datetime
        :return: DagRun corresponding to the given dag_id and execution date
            if one exists. None otherwise.
        :rtype: airflow.models.DagRun
        """
        return (
            session.query(DagRun)
            .filter(
                DagRun.dag_id == dag_id,
                DagRun.external_trigger == False,  
                DagRun.execution_date == execution_date,
            )
            .first()
        )
    @property
    def is_backfill(self) -> bool: