**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
"""The previous DagRun, if there is one"""
        filters = [
            DagRun.dag_id == self.dag_id,
            DagRun.execution_date < self.execution_date,
        ]
        if state is not None:
            filters.append(DagRun.state == state)
        return session.query(DagRun).filter(*filters).order_by(DagRun.execution_date.desc()).first()
    @provide_session
    def get_previous_scheduled_dagrun(self, session: Session = None) -> Optional['DagRun']:
        """The previous, SCHEDULED DagRun, if there is one"""
        dag = self.get_dag()
        return (
            session.query(DagRun)
            .filter(
                DagRun.dag_id == self.dag_id,
                DagRun.execution_date == dag.previous_schedule(self.execution_date),
            )
            .first()
        )
    @provide_session
    def update_state(