handle nonexistent users in can_view

nomen/notifications.py

@@ -42,6 +42,10 @@ async def in_thread(member, thread):
 
 
 async def can_view(ctx, member) -> bool:
+    # Nonexistent members cannot view
+    if member is None:
+        return False
+
     if ctx.channel.type == ChannelType.private_thread and not await in_thread(member, ctx.channel):
         return False