better logic in handling dead threads

Author: kyb3r

core/thread.py

@@ -657,7 +657,7 @@ async def find(
 
         try:
             thread = self.cache[recipient_id]
-            if not self.bot.get_channel(thread.channel.id):  # deleted channel
+            if not thread.channel or not self.bot.get_channel(thread.channel.id):
                 self.bot.loop.create_task(
                     thread.close(
                         closer=self.bot.user, silent=True, delete_channel=False