**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
"""
        DR = DagRun
        qry = session.query(DR)
        dag_ids = [dag_id] if isinstance(dag_id, str) else dag_id
        if dag_ids:
            qry = qry.filter(DR.dag_id.in_(dag_ids))
        if run_id:
            qry = qry.filter(DR.run_id == run_id)
        if execution_date:
            if isinstance(execution_date, list):
                qry = qry.filter(DR.execution_date.in_(execution_date))
            else:
                qry = qry.filter(DR.execution_date == execution_date)
        if execution_start_date and execution_end_date:
            qry = qry.filter(DR.execution_date.between(execution_start_date, execution_end_date))
        elif execution_start_date:
            qry = qry.filter(DR.execution_date >= execution_start_date)
        elif execution_end_date:
            qry = qry.filter(DR.execution_date <= execution_end_date)
        if state:
            qry = qry.filter(DR.state == state)
        if external_trigger is not None:
            qry = qry.filter(DR.external_trigger == external_trigger)
        if run_type:
            qry = qry.filter(DR.run_type == run_type)
        if no_backfills:
            qry = qry.filter(DR.run_type != DagRunType.BACKFILL_JOB)
        return qry.order_by(DR.execution_date).all()
    @staticmethod
    def generate_run_id(run_type: DagRunType, execution_date: datetime) -> str: