Handle channel deletion without audit log perms

Author: kyb3r

core/thread.py

@@ -635,6 +635,14 @@ async def find(self, *,
 
         try:
             thread = self.cache[recipient_id]
+            if not self.bot.get_channel(thread.channel.id): # deleted channel
+                self.bot.loop.create_task(
+                    thread.close(
+                        closer=self.bot.user, 
+                        silent=True, 
+                        delete_channel=False
+                        ))
+                thread = None
         except KeyError:
             channel = discord.utils.get(
                 self.bot.modmail_guild.text_channels,