Fix modmail reply bug

(cherry picked from commit praw-dev/praw@5984516)

CHANGES.rst

@@ -10,6 +10,10 @@ Unreleased
 
 - Drop asyncio_extras dependency, use contextlib.asynccontextmanager instead.
 
+**Fixed**
+
+- An issue with replying to a modmail conversation results in a error.
+
 7.7.0 (2023/02/25)
 ------------------
 

asyncpraw/models/reddit/modmail.py

@@ -272,9 +272,15 @@ async def reply(
         response = await self._reddit.post(
             API_PATH["modmail_conversation"].format(id=self.id), data=data
         )
-        message_id = response["conversation"]["objIds"][-1]["id"]
-        message_data = response["messages"][message_id]
-        return self._reddit._objector.objectify(message_data)
+        if isinstance(response, dict):
+            # Reddit recently changed the response format, so we need to handle both in case they change it back
+            message_id = response["conversation"]["objIds"][-1]["id"]
+            message_data = response["messages"][message_id]
+            return self._reddit._objector.objectify(message_data)
+        else:
+            for message in response.messages:
+                if message.id == response.obj_ids[-1]["id"]:
+                    return message
 
     async def unarchive(self):
         """Unarchive the conversation.