**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
self,
        dag_id: Optional[str] = None,
        run_id: Optional[str] = None,
        execution_date: Optional[datetime] = None,
        start_date: Optional[datetime] = None,
        external_trigger: Optional[bool] = None,
        conf: Optional[Any] = None,
        state: Optional[str] = None,
        run_type: Optional[str] = None,
        dag_hash: Optional[str] = None,
        creating_job_id: Optional[int] = None,
    ):
        self.dag_id = dag_id
        self.run_id = run_id
        self.execution_date = execution_date
        self.start_date = start_date
        self.external_trigger = external_trigger
        self.conf = conf or {}
        self.state = state
        self.run_type = run_type
        self.dag_hash = dag_hash
        self.creating_job_id = creating_job_id
        super().__init__()
    def __repr__(self):