You are probably trying to access something that is no longer available once your routine is being given command.
Most of the notifiers are firing your routine before the action is taken (removed track or row in pattern or whatever) and your routine then enumerates then current content of the pattern contents and when your routine wants to provoke an action on it, the action that caused your notifier to fire your routine, has being executed meanwhile and removed a last track or line causing your routine perhaps attempting to access something that is no longer there.
You may need this solution:
http://www.renoise.c…335#entry251335