This is similar to what I did, as a challenge, here: Loop of flashing light inside a timer - #2 by rlkoshak. This might be useful.
The error is complaining about the calls to substring. It’s telling you that Channel_select.state.toString is the empty String. Log value of Channel_select.state.toString and follow the state changes to that Item in events.log to see whether, when, and why it’s being set to the empty String.