
**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

airflow/operators/dagrun_operator.py
#
#
#
import datetime
import time
from typing import Dict, List, Optional, Union
from airflow.api.common.experimental.trigger_dag import trigger_dag
from airflow.exceptions import AirflowException, DagNotFound, DagRunAlreadyExists
from airflow.models import BaseOperator, BaseOperatorLink, DagBag, DagModel, DagRun
from airflow.utils import timezone
from airflow.utils.decorators import apply_defaults
from airflow.utils.helpers import build_airflow_url_with_query
from airflow.utils.state import State
from airflow.utils.types import DagRunType
class TriggerDagRunLink(BaseOperatorLink):
    """
    Operator link for TriggerDagRunOperator. It allows users to access
    DAG triggered by task using TriggerDagRunOperator.
    """
    name = 'Triggered DAG'
    def get_link(self, operator, dttm):
        query = {"dag_id": operator.trigger_dag_id, "execution_date": dttm.isoformat()}
        return build_airflow_url_with_query(query)
class TriggerDagRunOperator(BaseOperator):