Return 500 for the case of root_room_entry being None

Author: kfiven

synapse/handlers/room_summary.py

@@ -204,7 +204,14 @@ async def _get_room_hierarchy(
                 False,
             )
             root_room_entry = room_hierarchy[0]
-            if not root_room_entry or not await self._is_remote_room_accessible(
+            if not root_room_entry:
+                raise UnstableSpecAuthError(
+                    500,
+                    "Failed to get room preview for %s"
+                    % (requested_room_id),
+                    errcode=Codes.UNKNOWN,
+                )
+            if not await self._is_remote_room_accessible(
                 requester, requested_room_id, root_room_entry.room
             ):
                 raise UnstableSpecAuthError(