Handle the edge case when a subscription group has been deleted, but the the Subscribable entry has not been cleaned up. Fixes bug #11456.

docs/changelog/7.x.x.txt

@@ -12,6 +12,7 @@
  - fixed #11458: Use packed template removes wanted whitespace
  - fixed #11459: "default template" warning after upgrade to 7.8.14
  - added #11460: Override the search root asset. (United Knowledge)
+ - fixed #11456: Delete a wiki (Reopen #11446 - subscription groups)
 
 7.8.13
  - fixed #11418: confusing typ-o in gotcha

lib/WebGUI/AssetAspect/Subscribable.pm

@@ -428,7 +428,7 @@ sub purge {
     my $options = shift;
 
     my $group   = $self->getSubscriptionGroup();
-    $group->delete;
+    $group->delete if $group;
     my $success = $self->next::method($options);
 
     return $success;