What version are you running?
3.0.3
What's the URL of the page containing the problem?
http://localhost/account/login/
What steps will reproduce the problem?

Configure ldap in admin settings
Do not install python-ldap package
Try to login


What is the expected output? What do you see instead?
Expected: No login and a warning for the user. Also a logging of existing warning in line 607.
 if ldap is None:
        logging.error('Attempted to look up user "%s" in LDAP, but the '
                      'python-ldap package is not installed!',
                      username)
        return None


See: Error 500 and following stacktrace.

What operating system are you using? What browser?
Linux, Firefox
Please provide any additional information below.
2018-03-01 07:17:26,393 - ERROR - None - AnonymousUser - /account/login/ - djblets.log.middleware - Exception thrown for user AnonymousUser at http://localhost/account/login/
'NoneType' object has no attribute 'initialize'

Traceback (most recent call last):

  File "/var/www/dist/lib/python2.7/site-packages/django/core/handlers/base.py", line 112, in get_response

    response = wrapped_callback(request, callback_args, callback_kwargs)

  File "/var/www/dist/lib/python2.7/site-packages/django/views/decorators/debug.py", line 75, in sensitive_post_parameters_wrapper

    return view(request, *args, kwargs)

  File "/var/www/dist/lib/python2.7/site-packages/django/utils/decorators.py", line 99, in _wrapped_view

    response = view_func(request, args, kwargs)

  File "/var/www/dist/lib/python2.7/site-packages/django/views/decorators/cache.py", line 52, in _wrapped_view_func

    response = view_func(request, *args, kwargs)

  File "/var/www/dist/lib/python2.7/site-packages/django/contrib/auth/views.py", line 36, in login

    if form.is_valid():

  File "/var/www/dist/lib/python2.7/site-packages/django/forms/forms.py", line 129, in is_valid

    return self.is_bound and not bool(self.errors)

  File "/var/www/dist/lib/python2.7/site-packages/django/forms/forms.py", line 121, in errors

    self.full_clean()

  File "/var/www/dist/lib/python2.7/site-packages/django/forms/forms.py", line 274, in full_clean

    self._clean_form()

  File "/var/www/dist/lib/python2.7/site-packages/django/forms/forms.py", line 300, in _clean_form

    self.cleaned_data = self.clean()

  File "/var/www/dist/lib/python2.7/site-packages/reviewboard/accounts/forms/auth.py", line 502, in clean

    self.cleaned_data = super(AuthenticationForm, self).clean()

  File "/var/www/dist/lib/python2.7/site-packages/django/contrib/auth/forms.py", line 189, in clean

    password=password)

  File "/var/www/dist/lib/python2.7/site-packages/django/contrib/auth/init.py", line 49, in authenticate

    user = backend.authenticate(**credentials)

  File "/var/www/dist/lib/python2.7/site-packages/reviewboard/accounts/backends.py", line 989, in authenticate

    for con in connections:

  File "/var/www/dist/lib/python2.7/site-packages/reviewboard/accounts/backends.py", line 931, in get_ldap_connections

    con = ldap.initialize(ldap_uri)

AttributeError: 'NoneType' object has no attribute 'initialize'