A bug caused by a typo causes execute_script event object to not have a script id

I've been working on using the new event listeners to have propagator start sending messages to our dev chat room. However, I noticed that when an execute_script event would fire, that the script_id property in the event object would be empty. Upon further inspection, turns out there is a typo in lib/Propagator.php that was causing problems with assigning the proper script id.