**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
"""
        Returns the task instance specified by task_id for this dag run
        :param task_id: the task id
        :type task_id: str
        :param session: Sqlalchemy ORM Session
        :type session: Session
        """
        return (
            session.query(TI)
            .filter(TI.dag_id == self.dag_id, TI.execution_date == self.execution_date, TI.task_id == task_id)
            .first()
        )
    def get_dag(self) -> "DAG":
        """
        Returns the Dag associated with this DagRun.
        :return: DAG
        """
        if not self.dag:
            raise AirflowException(f"The DAG (.dag) for {self} needs to be set")
        return self.dag
    @provide_session
    def get_previous_dagrun(self, state: Optional[str] = None, session: Session = None) -> Optional['DagRun']: