doClassValidation_(opt_newValue) {
    if (opt_newValue === null || opt_newValue === undefined) {
      return null;
    }
    const note = this.valueToNote(opt_newValue);
    if (note) {
      return opt_newValue;
    }
    return null;
  }
Ensure that only a valid value may be entered.
@param {*} opt_newValue The input value.
@returns {*} A valid value, or null if invalid.