Adds an event handler that runs when the multi-state box moves to a new state.
onChange is fired when the multi-state box moves to a new state. This occurs
when the changeState() function is called.
eventHandler(event: Event): void The name of the function or
the function expression to run when the multi-state box moves to a new state.
This method doesn’t return any custom errors, but may return standard errors. Learn more about standard Wix errors.