**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
return self._state
    def set_state(self, state):
        if self._state != state:
            self._state = state
            self.end_date = timezone.utcnow() if self._state in State.finished else None
    @declared_attr
    def state(self):
        return synonym('_state', descriptor=property(self.get_state, self.set_state))
    @provide_session
    def refresh_from_db(self, session: Session = None):
        """
        Reloads the current dagrun from the database
        :param session: database session
        :type session: Session
        """
        DR = DagRun
        exec_date = func.cast(self.execution_date, DateTime)
        dr = (
            session.query(DR)
            .filter(
                DR.dag_id == self.dag_id,
                func.cast(DR.execution_date, DateTime) == exec_date,
                DR.run_id == self.run_id,
            )
            .one()
        )
        self.id = dr.id
        self.state = dr.state
    @classmethod
    def next_dagruns_to_examine(