Set pk of page in PageRevision.as_page_object

Otherwise, Django sees it as unsaved and doesn't allow any foreign keys to point to it

wagtail/wagtailcore/models.py

@@ -1085,6 +1085,7 @@ class PageRevision(models.Model):
 
         # Override the possibly-outdated tree parameter fields from this revision object
         # with up-to-date values
+        obj.pk = self.page.pk
         obj.path = self.page.path
         obj.depth = self.page.depth
         obj.numchild = self.page.numchild