Check if a user is in the room before sending a PowerLevel event on their behalf

changelog.d/9235.bugfix

@@ -0,0 +1 @@
+Fix a bug in the `make_room_admin` admin API where it failed if the admin with the greatest power level was not in the room. Contributed by Pankaj Yadav.

synapse/rest/admin/rooms.py

@@ -431,7 +431,17 @@ async def on_POST(self, request, room_identifier):
             if not admin_users:
                 raise SynapseError(400, "No local admin user in room")
 
-            admin_user_id = admin_users[-1]
+            admin_user_id = None
+
+            for admin_user in reversed(admin_users):
+                if room_state.get((EventTypes.Member, admin_user)):
+                    admin_user_id = admin_user
+                    break
+
+            if not admin_user_id:
+                raise SynapseError(
+                    400, "No local admin user in room",
+                )
 
             pl_content = power_levels.content
         else: