**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
nonlocal node_count
            node_count += 1
            visited.add(task)
            task_id = task.task_id
            node = {
                'name': task.task_id,
                'instances': [encode_ti(task_instances.get((task_id, d))) for d in dates],
                'num_dep': len(task.downstream_list),
                'operator': task.task_type,
                'retries': task.retries,
                'owner': task.owner,
                'ui_color': task.ui_color,
            }
            if task.downstream_list:
                children = [
                    recurse_nodes(t, visited)
                    for t in task.downstream_list
                    if node_count < node_limit or t not in visited
                ]
                
                
                
                if task.task_id not in expanded:
                    children_key = 'children'
                    expanded.add(task.task_id)
                else:
                    children_key = "_children"
                node[children_key] = children