When previewing a page creation, populate url_path.

This ensures that Page.dummy_request can infer a sensible hostname rather than falling back on example.com, which fails when ALLOWED_HOSTS is enforced.

wagtail/wagtailadmin/views/pages.py

@@ -349,6 +349,10 @@ def preview_on_create(request, content_type_app_name, content_type_model_name, p
     if form.is_valid():
         form.save(commit=False)
 
+        # ensure that our unsaved page instance has a suitable url set
+        parent_page = get_object_or_404(Page, id=parent_page_id).specific
+        page.set_url_path(parent_page)
+
         # This view will generally be invoked as an AJAX request; as such, in the case of
         # an error Django will return a plaintext response. This isn't what we want, since
         # we will be writing the response back to an HTML page regardless of success or