**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
for task in dag.tasks:
            task_id = task.task_id
            y_points[task_id] = []
            x_points[task_id] = []
            for ti in tis:
                ts = ti.execution_date
                if dag.schedule_interval and dag.following_schedule(ts):
                    ts = dag.following_schedule(ts)
                if ti.end_date:
                    dttm = wwwutils.epoch(ti.execution_date)
                    secs = (ti.end_date - ts).total_seconds()
                    x_points[task_id].append(dttm)
                    y_points[task_id].append(secs)
        
        
        y_unit = infer_time_unit([d for t in y_points.values() for d in t])
        
        chart.create_y_axis('yAxis', format='.02f', custom_format=False, label=f'Landing Time ({y_unit})')
        chart.axislist['yAxis']['axisLabelDistance'] = '-15'
        for task_id in x_points:
            chart.add_serie(
                name=task_id,
                x=x_points[task_id],
                y=scale_time_units(y_points[task_id], y_unit),
            )
        dates = sorted({ti.execution_date for ti in tis})
        max_date = max(ti.execution_date for ti in tis) if dates else None
        session.commit()
        form = DateTimeWithNumRunsForm(
            data={