**Apache Airflow version**: 2.0.0b2


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): N/A

**Environment**: 

- **Cloud provider or hardware configuration**: Ubuntu 20.04 Guest/Windows Server 2012 R2 Host
- **OS** (e.g. from /etc/os-release): Ubuntu 20.04.1 LTS
- **Kernel** (e.g. `uname -a`): Linux  5.4.0-1031-azure #32-Ubuntu SMPx86_64  GNU/Linux
- **Install tools**:  Pip 20.3b1, Ansible Template - https://github.com/jrd281/ansible-role-airflow/tree/airflowdbg-1
- **Others**: Python 3.8

**What happened**:  I was looking at the taskinstance list and then I clicked the edit icon.  After clicking, an error page was displayed.

<!-- (please include exact error messages if you can) -->

**What you expected to happen**: I expected to be whisked to an editing UI

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

**How to reproduce it**: 

1. Visit your installation's home page
2. Click the success bucket under the `Recent Tasks` heading
    <img width="261" alt="AirflowDebug#1" src="https://user-images.githubusercontent.com/848305/99696511-c5462b00-2a5c-11eb-92f1-088347b657a2.png">
3. Click the `Edit` button for one of your tasks
    <img width="285" alt="AirflowDebug#2" src="https://user-images.githubusercontent.com/848305/99696788-1524f200-2a5d-11eb-9127-5a308a8ba14c.png">
4. Watch the system throw an error.
    <img width="355" alt="AirflowDebug#3" src="https://user-images.githubusercontent.com/848305/99697050-57e6ca00-2a5d-11eb-93f1-1e88aaf59c63.png">

**Anything else we need to know**:

<details><summary>UI Error Page</summary>
Something bad has happened.
Please consider letting us know by creating a bug report using GitHub.

Python version: 3.8.5
Airflow version: 2.0.0b2
Node: REDACTED
-------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/security/decorators.py", line 109, in wraps
    return f(self, *args, **kwargs)
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/views.py", line 606, in edit
    return self.render_template(
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/baseviews.py", line 280, in render_template
    return render_template(
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask/templating.py", line 137, in render_template
    return _render(
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask/templating.py", line 120, in _render
    rv = template.render(context)
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/jinja2/environment.py", line 1090, in render
    self.environment.handle_exception()
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/jinja2/environment.py", line 832, in handle_exception
    reraise(*rewrite_traceback_stack(source=source))
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/jinja2/_compat.py", line 28, in reraise
    raise value.with_traceback(tb)
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/templates/appbuilder/general/model/edit.html", line 2, in top-level template code
    {% import 'appbuilder/general/lib.html' as lib %}
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/templates/appbuilder/base.html", line 1, in top-level template code
    {% extends base_template %}
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/airflow/www/templates/airflow/master.html", line 20, in top-level template code
    {% extends 'appbuilder/baselayout.html' %}
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/templates/appbuilder/baselayout.html", line 2, in top-level template code
    {% import 'appbuilder/baselib.html' as baselib %}
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/templates/appbuilder/init.html", line 46, in top-level template code
    {% block body %}
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/templates/appbuilder/baselayout.html", line 19, in block "body"
    {% block content %}
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/templates/appbuilder/general/model/edit.html", line 23, in block "content"
    {% block edit_form %}
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/templates/appbuilder/general/model/edit.html", line 25, in block "edit_form"
    {{ widgets.get('edit')(form_action=form_action)|safe }}
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/widgets.py", line 37, in __call__
    return template.render(args)
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/jinja2/environment.py", line 1090, in render
    self.environment.handle_exception()
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/jinja2/environment.py", line 832, in handle_exception
    reraise(*rewrite_traceback_stack(source=source))
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/jinja2/_compat.py", line 28, in reraise
    raise value.with_traceback(tb)
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/templates/appbuilder/general/widgets/form.html", line 45, in top-level template code
    {{ lib.render_field(field, begin_sep_label, end_sep_label, begin_sep_field, end_sep_field) }}
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/jinja2/runtime.py", line 679, in _invoke
    rv = self._func(*arguments)
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/templates/appbuilder/general/lib.html", line 237, in template
    {{ field(**kwargs)|safe }}
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/wtforms/fields/core.py", line 160, in __call__
    return self.meta.render_field(self, kwargs)
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/wtforms/meta.py", line 56, in render_field
    return field.widget(field, **render_kw)
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/fieldwidgets.py", line 176, in __call__
    return super(Select2ManyWidget, self).__call__(field, **kwargs)
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/wtforms/widgets/core.py", line 300, in __call__
    for val, label, selected in field.iter_choices():
  File "/var/lib/airflow/venv/lib/python3.8/site-packages/flask_appbuilder/fields.py", line 209, in iter_choices
    yield (pk, self.get_label(obj), obj in self.data)
TypeError: argument of type 'DagRun' is not iterable
</details>

**How often does this problem occur?:** Every time I click the `Edit` icon

airflow/www/views.py
"""Clears the state."""
        try:
            count = 0
            cleared_ti_count = 0
            dag_to_tis = {}
            for dr in session.query(DagRun).filter(DagRun.id.in_([dagrun.id for dagrun in drs])).all():
                count += 1
                dag = current_app.dag_bag.get_dag(dr.dag_id)
                tis_to_clear = dag_to_tis.setdefault(dag, [])
                tis_to_clear += dr.get_task_instances()
            for dag, tis in dag_to_tis.items():
                cleared_ti_count += len(tis)
                models.clear_task_instances(tis, session, dag=dag)
            flash(
                "{count} dag runs and {altered_ti_count} task instances "
                "were cleared".format(count=count, altered_ti_count=cleared_ti_count)
            )
        except Exception:  
            flash('Failed to clear state', 'error')
        return redirect(self.get_default_url())
class LogModelView(AirflowModelView):
    """View to show records from Log table"""
    route_base = '/log'
    datamodel = AirflowModelView.CustomSQLAInterface(Log)  
    class_permission_name = permissions.RESOURCE_AUDIT_LOG