**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
"""Show confirmation page for marking tasks as success or failed."""
        args = request.args
        dag_id = args.get('dag_id')
        task_id = args.get('task_id')
        execution_date = args.get('execution_date')
        state = args.get('state')
        upstream = to_boolean(args.get('upstream'))
        downstream = to_boolean(args.get('downstream'))
        future = to_boolean(args.get('future'))
        past = to_boolean(args.get('past'))
        try:
            dag = current_app.dag_bag.get_dag(dag_id)
        except airflow.exceptions.SerializedDagNotFound:
            flash(f'DAG {dag_id} not found', "error")
            return redirect(request.referrer or url_for('Airflow.index'))
        try:
            task = dag.get_task(task_id)
        except airflow.exceptions.TaskNotFound:
            flash(f"Task {task_id} not found", "error")
            return redirect(request.referrer or url_for('Airflow.index'))
        task.dag = dag
        if state not in (
            'success',
            'failed',
        ):
            flash(f"Invalid state {state}, must be either 'success' or 'failed'", "error")
            return redirect(request.referrer or url_for('Airflow.index'))
        latest_execution_date = dag.get_latest_execution_date()
        if not latest_execution_date:
            flash(f"Cannot mark tasks as {state}, seem that dag {dag_id} has never run", "error")