src/main/java/com/github/oxo42/stateless4j/StateMachine.java
public void onUnhandledTrigger(Action3<S, T, Object[]> unhandledTriggerAction) {
        if (unhandledTriggerAction == null) {
            throw new IllegalStateException("unhandledTriggerAction");
        }
        this.unhandledTriggerAction = unhandledTriggerAction;
    }
public void onUnhandledTrigger(Action3<S, T, Object[]> unhandledTriggerAction) {
        if (unhandledTriggerAction == null) {
            throw new IllegalStateException("unhandledTriggerAction");
        }
        this.unhandledTriggerAction = unhandledTriggerAction;
    }
Override the default behaviour of throwing an exception when an unhandled trigger is fired

@param unhandledTriggerAction An action to call with state, trigger and params when an unhandled trigger is fired