**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
#
#
#
#
import copy
import itertools
import json
import logging
import math
import socket
import sys
import traceback
from collections import defaultdict
from datetime import datetime, timedelta
from json import JSONDecodeError
from typing import Dict, List, Optional, Tuple
from urllib.parse import parse_qsl, unquote, urlencode, urlparse
import lazy_object_proxy
import nvd3
import sqlalchemy as sqla
import yaml
from flask import (
    Markup,
    Response,
    abort,
    current_app,
    escape,