Don't try to track non-existant channels

ahtcg_bot.py

@@ -101,8 +101,14 @@ class ArkhamDBUpdater(discord.Client):
             logging.info(f"  - {guild.name}")
         logging.info("------")
 
-        for channel_id in self.channel_list:
+        for channel_id in list(self.channel_list):
             channel = self.get_channel(channel_id)
+            if channel is None:
+                print(
+                    f"channel {channel_id} does not exist, removing it from tracking!"
+                )
+                self.channel_list.remove(channel_id)
+                continue
             permissions = channel.permissions_for(channel.guild.me)
             missing_permissions = (
                 permissions & expected_permissions