Fix third-party event modules for `check_visibility_can_be_modified` check

PR #8292 tried to maintain backwards compat with modules which don't provide a
`check_visibility_can_be_modified` method, but the tests weren't being run,
and the check didn't work.

1 files changed, 3 insertions, 1 deletions

diff --git a/synapse/events/third_party_rules.py b/synapse/events/third_party_rules.py

index fed459198a..1ca77519d5 100644
--- a/synapse/events/third_party_rules.py
+++ b/synapse/events/third_party_rules.py
@@ -131,7 +131,9 @@ class ThirdPartyEventRules:
         if self.third_party_rules is None:
             return True
 
-        check_func = getattr(self.third_party_rules, "check_visibility_can_be_modified")
+        check_func = getattr(
+            self.third_party_rules, "check_visibility_can_be_modified", None
+        )
         if not check_func or not isinstance(check_func, Callable):
             return True