**Apache Airflow version**: 2.1.0

**Browsers**: Chrome and Firefox

**What happened**:

Before upgrading to 2.1.0

![before](https://user-images.githubusercontent.com/14293802/120359517-c1ca1100-c2d5-11eb-95ba-58ccc0a3ac37.png)

After upgrading to 2.1.0

![after](https://user-images.githubusercontent.com/14293802/120359528-c4c50180-c2d5-11eb-8e04-f34846ea2736.png)

**What you expected to happen**:

Show/Edit/Delete under Security -> Users are available

<!-- What do you think went wrong? -->

**How to reproduce it**:

Go to Security -> Users (as an admin of course) 
airflow/www/views.py
'url': None,
                    'error': f"can't find dag {dag} or task_id {task_id}",
                }
            )
            response.status_code = 404
            return response
        task = dag.get_task(task_id)
        try:
            url = task.get_extra_links(dttm, link_name)
        except ValueError as err:
            response = jsonify({'url': None, 'error': str(err)})
            response.status_code = 404
            return response
        if url:
            response = jsonify({'error': None, 'url': url})
            response.status_code = 200
            return response
        else:
            response = jsonify({'url': None, 'error': f'No URL found for {link_name}'})
            response.status_code = 404
            return response
    @expose('/object/task_instances')
    @auth.has_access(
        [
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
        ]
    )
    @action_logging
    def task_instances(self):