runtimer.js

function* runTimer(getState) {
  
  while(yield take('START')) {
    while(true) {
      const {stop, tick} = yield race({
        stop   : take('STOP'),
        tick : call(wait, ONE_SECOND);
      })
      if ( !stop ) {
        yield put(actions.tick());
      } else {
        break;
      }
    }
  }

}

I like @mrsoto's suggestion. A little one-liner is more readable:

if (stop) { break }
yield put(actions.tick())