getReactiveState = (stateKey) => {
  const stateObject = reactiveStates[stateKey];
  if (!stateObject) {
    throw new Error(`There is no reactive state with stateKey ${stateKey}`);
  }

  return stateObject;
}
Return a reactive state previously created
@param {string} stateKey The key of the desired reactive state
@returns {ReactiveState}
@throws Will throw an error if there is no reactive state with the given key