Fix TypeError due to None LEARNING_MICROFRONTEND_URL setting value

We observed the following error in production:

    lms_1             | 2020-06-22 14:03:10,220 ERROR 10 [django.request]
    [user 4] log.py:228 - Internal Server Error:
    /xblock/block-v1:edX+DemoX+Demo_Course+type@html+block@6bcccc2d7343416e9e03
    fd7325b2f232
    lms_1             | Traceback (most recent call last):
    lms_1             |   File
    "/openedx/venv/lib/python3.5/site-packages/django/core/handlers/exception.py",
    line 34, in inner
    lms_1             |     response = get_response(request)
    lms_1             |   File
    "/openedx/venv/lib/python3.5/site-packages/django/core/handlers/base.py",
    line 115, in _get_response
    lms_1             |     response =
    self.process_exception_by_middleware(e, request)
    lms_1             |   File
    "/openedx/venv/lib/python3.5/site-packages/django/core/handlers/base.py",
    line 113, in _get_response
    lms_1             |     response = wrapped_callback(request,
    *callback_args, **callback_kwargs)
    lms_1             |   File
    "/openedx/venv/lib/python3.5/site-packages/django/views/decorators/http.py",
    line 40, in inner
    lms_1             |     return func(request, *args, **kwargs)
    lms_1             |   File
    "/openedx/edx-platform/common/djangoapps/util/views.py", line 66, in
    inner
    lms_1             |     response = view_func(request, *args, **kwargs)
    lms_1             |   File
    "/openedx/venv/lib/python3.5/site-packages/django/views/decorators/clickjacking.py",
    line 50, in wrapped_view
    lms_1             |     resp = view_func(*args, **kwargs)
    lms_1             |   File
    "/openedx/venv/lib/python3.5/site-packages/django/utils/decorators.py",
    line 142, in _wrapped_view
    lms_1             |     response = view_func(request, *args, **kwargs)
    lms_1             |   File
    "/openedx/edx-platform/lms/djangoapps/courseware/views/views.py", line
    1683, in render_xblock
    lms_1             |     'is_learning_mfe':
    request.META.get('HTTP_REFERER',
    '').startswith(settings.LEARNING_MICROFRONTEND_URL),
    lms_1             | TypeError: startswith first arg must be str or a
    tuple of str, not NoneType

CHANGELOG.md

@@ -4,6 +4,7 @@ Note: Breaking changes between versions are indicated by "💥".
 
 ## Unreleased
 
+- [Bugfix] Fix TypeError on viewing xblock
 - [Bugfix] Fix authentication in Android mobile application
 
 ## v10.0.6 (2020-06-22)

tutor/templates/apps/openedx/settings/partials/common_lms.py

@@ -1,9 +1,10 @@
 {% include "apps/openedx/settings/partials/common_all.py" %}
 
 ######## Common LMS settings
-
 LOGIN_REDIRECT_WHITELIST = ["{{ CMS_HOST }}"]
-LEARNING_MICROFRONTEND_URL = None
+
+# This url must not be None and should not be used anywhere
+LEARNING_MICROFRONTEND_URL = "http://learn.openedx.org"
 
 # Fix media files paths
 PROFILE_IMAGE_BACKEND["options"]["location"] = os.path.join(