title: Airflow can not enter the /admin page after initdb
body: <p>I wanted to clear all example dags, so that I ran the command <code>airflow initdb</code>. However, I was not able to enter the admin page after that, as there was something wrong:</p>
  
  <pre><code>  File "/data/software/miniconda3/lib/python3.7/site-packages/flask_admin/menu.py", line 126, in is_accessible
      return self._view.is_accessible()
    File "/data/software/miniconda3/lib/python3.7/site-packages/airflow/www/utils.py", line 93, in is_accessible
      (not current_user.is_anonymous and current_user.is_superuser())
    File "/data/software/miniconda3/lib/python3.7/site-packages/airflow/contrib/auth/backends/password_auth.py", line 114, in is_superuser
      return hasattr(self, 'user') and self.user.is_superuser()
  AttributeError: 'NoneType' object has no attribute 'is_superuser'
  </code></pre>
  
  <p>I tried to create the same superuser as before, but it still got the same error.</p>
  
  <p>Is there any idea how to fix it?
  Thanks in advance.</p>