**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/models/dag.py
"""
        Get the next execution date after the given ``date_last_automated_dagrun``, according to
        schedule_interval, start_date, end_date etc.  This doesn't check max active run or any other
        "concurrency" type limits, it only performs calculations based on the various date and interval fields
        of this dag and it's tasks.
        :param date_last_automated_dagrun: The execution_date of the last scheduler or
            backfill triggered run for this dag
        :type date_last_automated_dagrun: pendulum.Pendulum
        """
        if not self.schedule_interval or self.is_subdag:
            return None
        
        if self.schedule_interval == '@once' and date_last_automated_dagrun:
            return None
        
        if not (self.catchup or self.schedule_interval == '@once'):
            
            
            
            now = timezone.utcnow()
            next_start = self.following_schedule(now)
            last_start = self.previous_schedule(now)
            if next_start <= now or isinstance(self.schedule_interval, timedelta):
                new_start = last_start
            else: