The spec for this action:
https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf#G11.2111850
An example of a form using this action:
https://www2.illinois.gov/rev/forms/reg/Documents/reg-1-mr.pdf
this._currentValueIndices = indices[0];
        this._value = this._items[this._currentValueIndices];
      }
    }
    this._send({ id: this._id, indices });
  }
  get fillColor() {
    return this._fillColor;
  }
  set fillColor(color) {
    if (Color._isValidColor(color)) {
      this._fillColor = color;
    }
  }
  get bgColor() {
    return this.fillColor;
  }
  set bgColor(color) {
    this.fillColor = color;
  }
  get numItems() {
    if (!this._isChoice) {
      throw new Error("Not a choice widget");
    }
    return this._items.length;
  }
  set numItems(_) {
    throw new Error("field.numItems is read-only");
  }
  get strokeColor() {