Fix AttributeError in listing_view endpoint

Fixes #3967

If there was no site configured, the listing views in the API would
break with an AttributeError.

Now, the following happens if there is no site record:
 - In the Admin API, all pages are returned but links in the response do
not include a domain name
 - In the Public API, no pages are returned

wagtail/api/v2/endpoints.py

@@ -396,7 +396,11 @@ class PagesAPIEndpoint(BaseAPIEndpoint):
         queryset = queryset.public().live()
 
         # Filter by site
-        queryset = queryset.descendant_of(request.site.root_page, inclusive=True)
+        if request.site:
+            queryset = queryset.descendant_of(request.site.root_page, inclusive=True)
+        else:
+            # No sites configured
+            queryset = queryset.none()
 
         return queryset
 

wagtail/api/v2/tests/test_pages.py

@@ -11,7 +11,7 @@ from django.urls import reverse
 from wagtail.api.v2 import signal_handlers
 from wagtail.tests.demosite import models
 from wagtail.tests.testapp.models import StreamPage
-from wagtail.wagtailcore.models import Page
+from wagtail.wagtailcore.models import Page, Site
 
 
 def get_total_page_count():
@@ -753,6 +753,14 @@ class TestPageListing(TestCase):
         self.assertEqual(response['Content-type'], 'application/json')
         self.assertEqual(content['meta']['total_count'], 0)
 
+    # REGRESSION TESTS
+
+    def test_issue_3967(self):
+        # The API crashed whenever the listing view was called without a site configured
+        Site.objects.all().delete()
+        response = self.get_response()
+        self.assertEqual(response.status_code, 200)
+
 
 class TestPageDetail(TestCase):
     fixtures = ['demosite.json']

wagtail/api/v2/utils.py

@@ -12,7 +12,7 @@ class BadRequestError(Exception):
 
 
 def get_base_url(request=None):
-    base_url = getattr(settings, 'WAGTAILAPI_BASE_URL', request.site.root_url if request else None)
+    base_url = getattr(settings, 'WAGTAILAPI_BASE_URL', request.site.root_url if request and request.site else None)
 
     if base_url:
         # We only want the scheme and netloc