
**Apache Airflow version**: 2.0.0b3


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**: MacOS with Docker

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

```
Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/airflow/models/taskinstance.py", line 984, in _run_raw_task
    result = task_copy.execute(context=context)
  File "/usr/local/airflow/plugins/custom_trigger_operator.py", line 138, in execute
    dag_run.execution_date,
UnboundLocalError: local variable 'dag_run' referenced before assignment
```

**What you expected to happen**:

The dag_run object doesn't exist in https://github.com/apache/airflow/blob/94ba200d42403d1067a63e2e6b158d893c2e8b5a/airflow/operators/dagrun_operator.py#L161
as the DagRunAlreadyExists exception is raised

**How to reproduce it**:

```
first_task = DummyOperator(...)

trigger_task = TriggerDagRunOperator(...
reset_dag_run=True,
wait_for_completion=True,
trigger_dag_id='my_dag_to_trigger'
...)

```
Run the DAG above once. It success. Clear the task first_task to run a second time the DAG and so trigger again my_dag_to_trigger, you will get the error.

**Anything else we need to know**:

Nope

tests/operators/test_dagrun_operator.py
def setUp(self):
        
        
        with tempfile.NamedTemporaryFile(mode="w", delete=False) as f:
            self._tmpfile = f.name
            f.write(DAG_SCRIPT)
            f.flush()
        with create_session() as session:
            session.add(DagModel(dag_id=TRIGGERED_DAG_ID, fileloc=self._tmpfile))
            session.commit()
        self.dag = DAG(TEST_DAG_ID, default_args={"owner": "airflow", "start_date": DEFAULT_DATE})
        dagbag = DagBag(f.name, read_dags_from_db=False, include_examples=False)
        dagbag.bag_dag(self.dag, root_dag=self.dag)
        dagbag.sync_to_db()
    def tearDown(self):