**Apache Airflow version**:
1.10.10

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

kubeenetes ver: 1.15.5

**Environment**:

- **Cloud provider or hardware configuration**: Used docker based Airflow Image
- **OS** (e.g. from /etc/os-release): Debian
- **Kernel** (e.g. `uname -a`): 4.19.76-linuxkit x86_64 Linux


**What happened**:

When user navigates to tree view of dag and keep on clicking on `Go` button at `number of runs` the` base date` cycle back to one dag run and in the end when there is no dag run left, it shows `invalid date`. In the end user gets below exception: 

```
                          ____/ (  (    )   )  \___
                         /( (  (  )   _    ))  )   )\
                       ((     (   )(    )  )   (   )  )
                     ((/  ( _(   )   (   _) ) (  () )  )
                    ( (  ( (_)   ((    (   )  .((_ ) .  )_
                   ( (  )    (      (  )    )   ) . ) (   )
                  (  (   (  (   ) (  _  ( _) ).  ) . ) ) ( )
                  ( (  (   ) (  )   (  ))     ) _)(   )  )  )
                 ( (  ( \ ) (    (_  ( ) ( )  )   ) )  )) ( )
                  (  (   (  (   (_ ( ) ( _    )  ) (  )  )   )
                 ( (  ( (  (  )     (_  )  ) )  _)   ) _( ( )
                  ((  (   )(    (     _    )   _) _(_ (  (_ )
                   (_((__(_(__(( ( ( |  ) ) ) )_))__))_)___)
                   ((__)        \\||lll|l||///          \_))
                            (   /(/ (  )  ) )\   )
                          (    ( ( ( | | ) ) )\   )
                           (   /(| / ( )) ) ) )) )
                         (     ( ((((_(|)_)))))     )
                          (      ||\(|(|)|/||     )
                        (        |(||(||)||||        )
                          (     //|/l|||)|\\ \     )
                        (/ / //  /|//||||\\  \ \  \ _)
-------------------------------------------------------------------------------
Node: c6059861e1e9
-------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/pendulum/parsing/parser.py", line 352, in _parse
    yearfirst=self._options['year_first']
  File "/usr/local/lib/python3.7/site-packages/dateutil/parser/_parser.py", line 1374, in parse
    return DEFAULTPARSER.parse(timestr, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/dateutil/parser/_parser.py", line 649, in parse
    raise ParserError("Unknown string format: %s", timestr)
dateutil.parser._parser.ParserError: Unknown string format: Invalid date
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.7/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/decorators.py", line 121, in wrapper
    return f(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/flask_appbuilder/security/decorators.py", line 109, in wraps
    return f(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/decorators.py", line 92, in view_func
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/decorators.py", line 56, in wrapper
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/utils/db.py", line 74, in wrapper
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/views.py", line 1406, in tree
    base_date = timezone.parse(base_date)
  File "/usr/local/lib/python3.7/site-packages/airflow/utils/timezone.py", line 180, in parse
    return pendulum.parse(string, tz=timezone or TIMEZONE)
  File "/usr/local/lib/python3.7/site-packages/pendulum/parser.py", line 75, in parse
    return Parser(**options).parse(text)
  File "/usr/local/lib/python3.7/site-packages/pendulum/parser.py", line 31, in parse
    parsed = super(Parser, self).parse(text)
  File "/usr/local/lib/python3.7/site-packages/pendulum/parsing/parser.py", line 297, in parse
    return self.normalize(self._parse(text))
  File "/usr/local/lib/python3.7/site-packages/pendulum/parsing/parser.py", line 355, in _parse
    raise ParserError('Invalid date string: {}'.format(text))
pendulum.parsing.exceptions.ParserError: Invalid date string: Invalid date
```

**What you expected to happen**:

User should not receive exception pendulum.parsing.exceptions.ParserError: Invalid date string: Invalid date 

**How to reproduce it**:

1. Create a sample dag/example dag with shorter scheduled interval 
2. Have 2-3 dag runs.
3. Navigate to tree view and click on `Go `button
4. Keep clicking on Go button till there are no dag runs remaining, you would get  `pendulum.parsing.exceptions.ParserError: Invalid date string: Invalid date `exception.

**Screenshot**:

![image](https://user-images.githubusercontent.com/60378152/93097661-78904d80-f6c3-11ea-8616-bf7ac5b17293.png)

![image](https://user-images.githubusercontent.com/60378152/93097682-81811f00-f6c3-11ea-941d-0e82e71f198a.png)

![image](https://user-images.githubusercontent.com/60378152/93097729-8e9e0e00-f6c3-11ea-884c-8ec6ba363fa4.png)


How often does this problem occur? Once? Every time etc?

Everytime


airflow/www/views.py
from airflow.jobs.scheduler_job import SchedulerJob
from airflow.models import Connection, DagModel, DagTag, Log, SlaMiss, TaskFail, XCom, errors
from airflow.models.baseoperator import BaseOperator
from airflow.models.dagcode import DagCode
from airflow.models.dagrun import DagRun, DagRunType
from airflow.models.taskinstance import TaskInstance
from airflow.security import permissions
from airflow.ti_deps.dep_context import DepContext
from airflow.ti_deps.dependencies_deps import RUNNING_DEPS, SCHEDULER_QUEUED_DEPS
from airflow.utils import json as utils_json, timezone
from airflow.utils.dates import infer_time_unit, scale_time_units
from airflow.utils.helpers import alchemy_to_dict
from airflow.utils.log.log_reader import TaskLogReader
from airflow.utils.session import create_session, provide_session
from airflow.utils.state import State
from airflow.version import version
from airflow.www import auth, utils as wwwutils
from airflow.www.decorators import action_logging, gzipped
from airflow.www.forms import (
    ConnectionForm,
    DagRunForm,
    DateTimeForm,
    DateTimeWithNumRunsForm,
    DateTimeWithNumRunsWithDagRunsForm,
)
from airflow.www.widgets import AirflowModelListWidget
PAGE_SIZE = conf.getint('webserver', 'page_size')
FILTER_TAGS_COOKIE = 'tags_filter'
FILTER_STATUS_COOKIE = 'dag_status_filter'
def get_safe_url(url):