I would recommend making a minimal example of the error. You should be able to get it down to a handful of isolated lines. This is a good general debugging tactic but one specific reason I recommend it is that the only sexp calling getLine relies (via line-info and mixer-info) on a fn get-current-mixer that does not seem to exist. It’s probably just a rename of get-current-mixer-info, but when mistakes like that crop up in an example it’s usually a good idea to take a step back and buckle down on your process.
Refactoring this code to a minimal example will probably also help by removing the layers of indirection in the arguments you’re passing to getLine. As it stands the values being passed to that method are hidden behind enough lets and wrapper functions (both of which seem unnecessary to me) to make it unclear what’s actually causing the error.