function get(entity, entitySelector) {
  if (!this.state[entity][entitySelector]) {
    return null;
  }
  return this.state[entity][entitySelector].get();
}
@description Return the value of a key in the store.
@param {string} entity - The type of entity we should get the value from.
@param {string} entitySelector - The selector to identify one entity.
@returns {any} Return the full state in store.
@example
stateManager.get('device', 'main-lamp');