throwIfDestroyed_() {
    if (this.isDestroyed()) {
      throw new shaka.util.Error(
          shaka.util.Error.Severity.CRITICAL,
          shaka.util.Error.Category.PLAYER,
          shaka.util.Error.Code.OBJECT_DESTROYED);
    }
  }
Throw if destroyed, to interrupt processes with a recognizable error.

@private