**Apache Airflow version**: v2.0.0.dev0 (latest master)

**Environment**:
- **OS**: Ubuntu 18.04.4 LTS
- **Others**: Python 3.6.9

**What happened**:

Selecting a task in the the **task instance list** (*http:localhost:8080/taskinstance/list/*) and **performing an Action** on it (e.g. *Set state to 'failed'*) yields an error.

Error message:

```
Something bad has happened.
Please consider letting us know by creating a bug report using Github.

Python version: 3.6.12
Airflow version: 2.0.0.dev0
Node: emma
-------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.6/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/airflow/.local/lib/python3.6/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/airflow/.local/lib/python3.6/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/airflow/.local/lib/python3.6/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/home/airflow/.local/lib/python3.6/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/airflow/.local/lib/python3.6/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/airflow/.local/lib/python3.6/site-packages/flask_appbuilder/views.py", line 686, in action_post
    self.datamodel.get(self._deserialize_pk_if_composite(pk)) for pk in pks
  File "/home/airflow/.local/lib/python3.6/site-packages/flask_appbuilder/views.py", line 686, in <listcomp>
    self.datamodel.get(self._deserialize_pk_if_composite(pk)) for pk in pks
  File "/home/airflow/.local/lib/python3.6/site-packages/flask_appbuilder/models/sqla/interface.py", line 870, in get
    query, _filters, select_columns=select_columns
  File "/home/airflow/.local/lib/python3.6/site-packages/flask_appbuilder/models/sqla/interface.py", line 324, in apply_all
    select_columns,
  File "/home/airflow/.local/lib/python3.6/site-packages/flask_appbuilder/models/sqla/interface.py", line 272, in _apply_inner_all
    query = self.apply_filters(query, inner_filters)
  File "/home/airflow/.local/lib/python3.6/site-packages/flask_appbuilder/models/sqla/interface.py", line 162, in apply_filters
    return filters.apply_all(query)
  File "/home/airflow/.local/lib/python3.6/site-packages/flask_appbuilder/models/filters.py", line 295, in apply_all
    query = flt.apply(query, value)
  File "/home/airflow/.local/lib/python3.6/site-packages/flask_appbuilder/models/sqla/filters.py", line 137, in apply
    query, field = get_field_setup_query(query, self.model, self.column_name)
  File "/home/airflow/.local/lib/python3.6/site-packages/flask_appbuilder/models/sqla/filters.py", line 40, in get_field_setup_query
    if not hasattr(model, column_name):
TypeError: hasattr(): attribute name must be string
```

**How to reproduce it**:

I wanted to take an Action on a task instance of a DAG with `schedule_interval=None`. I am attaching a minimal DAG file used for reproducing this error. 

<details>
<summary>DAG file</summary>

```
from airflow import DAG
from datetime import timedelta, datetime
from airflow.operators.bash_operator import BashOperator

dag = DAG(
    'simple_dag',
    default_args= {
        'owner': 'airflow',
        'depends_on_past': False,
        'retries' : 0,
        'start_date': datetime(1970, 1, 1),
        'retry_delay': timedelta(seconds=30),
    },
    description='',
    schedule_interval=None,
    catchup=False,
)

t1 = BashOperator(
    task_id='task1',
    bash_command='echo 1',
    dag=dag
)
```

</details>

**Anything else we need to know**:

Taking the same Action on the DagRun list *(http://localhost:8080/dagrun/list)* works.

Great project btw 🙌. Really enjoying using it.
airflow/www/utils.py
"""
    FAB does not know how to handle columns with leading underscores because
    they are not supported by WTForm. This hack will remove the leading
    '_' from the key to lookup the column names.
    """
    def __init__(self, obj, session=None):
        super().__init__(obj, session=session)
        def clean_column_names():
            if self.list_properties:
                self.list_properties = {
                    k.lstrip('_'): v for k, v in self.list_properties.items()}
            if self.list_columns:
                self.list_columns = {
                    k.lstrip('_'): v for k, v in self.list_columns.items()}
        clean_column_names()
    def is_utcdatetime(self, col_name):
        """Check if the datetime is a UTC one."""
        from airflow.utils.sqlalchemy import UtcDateTime
        if col_name in self.list_columns:
            obj = self.list_columns[col_name].type
            return isinstance(obj, UtcDateTime) or \
                isinstance(obj, sqla.types.TypeDecorator) and \
                isinstance(obj.impl, UtcDateTime)
        return False
    filter_converter_class = UtcAwareFilterConverter
FieldConverter.conversion_table = (
    (('is_utcdatetime', DateTimeWithTimezoneField, AirflowDateTimePickerWidget),) +
    FieldConverter.conversion_table
)