code: public void onUnhandledTrigger(final Action2<S, T> unhandledTriggerAction) {
          if (unhandledTriggerAction == null) {
              throw new IllegalStateException("unhandledTriggerAction");
          }
          this.unhandledTriggerAction = new Action3<S, T, Object[]>() {
              @Override
              public void doIt(S state, T trigger, Object[] arg3) {
                  unhandledTriggerAction.doIt(state, trigger);
              }
          };
      }
docs: Override the default behaviour of throwing an exception when an unhandled trigger is fired
  
  @param unhandledTriggerAction An action to call when an unhandled trigger is fired