Superset cannot start, reporting "AttributeError: 'NoneType' object has no attribute 'session' at startup"

I’m trying both Superset 0.20.6 and 0.28.1 (need to run them as containers, downloaded from dockerhub from https://hub.docker.com/r/amancevice/superset). At startup, both versions throw at some point an exception with the following error: AttributeError: 'NoneType' object has no attribute 'session'

Expected results

Superset start with no issues.

Actual results

Superset throws the following exception. Following is what I see using the 0.20.6 version.

superset    | Loaded your LOCAL configuration at [/etc/superset/superset_config.py]
superset    | Traceback (most recent call last):
superset    |   File "/usr/local/bin/superset", line 12, in <module>
superset    |     from superset.cli import manager
superset    |   File "/usr/local/lib/python3.5/dist-packages/superset/__init__.py", line 153, in <module>
superset    |     security_manager_class=app.config.get("CUSTOM_SECURITY_MANAGER"))
superset    |   File "/usr/local/lib/python3.5/dist-packages/flask_appbuilder/base.py", line 131, in __init__
superset    |     self.init_app(app, session)
superset    |   File "/usr/local/lib/python3.5/dist-packages/flask_appbuilder/base.py", line 157, in init_app
superset    |     self._add_admin_views()
superset    |   File "/usr/local/lib/python3.5/dist-packages/flask_appbuilder/base.py", line 254, in _add_admin_views
superset    |     self.sm.register_views()
superset    |   File "/usr/local/lib/python3.5/dist-packages/flask_appbuilder/security/sqla/manager.py", line 67, in register_views
superset    |     super(SecurityManager, self).register_views()
superset    |   File "/usr/local/lib/python3.5/dist-packages/flask_appbuilder/security/manager.py", line 443, in register_views
superset    |     category_label=_('Security'))
superset    |   File "/usr/local/lib/python3.5/dist-packages/flask_appbuilder/base.py", line 345, in add_view
superset    |     baseview = self._check_and_init(baseview)
superset    |   File "/usr/local/lib/python3.5/dist-packages/flask_appbuilder/base.py", line 294, in _check_and_init
superset    |     if baseview.datamodel.session is None:
superset    | AttributeError: 'NoneType' object has no attribute 'session'

How to reproduce the bug

Environment

• superset version: 0.20.6 or 0.28.1
• python version: 3 (coming with docker container)

Checklist

Make sure these boxes are checked before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven’t found one similar.

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