docs: @NonNull
    public RequestCreator placeholder(@DrawableRes int placeholderResId) {
      if (!setPlaceholder) {
        throw new IllegalStateException("Already explicitly declared as no placeholder.");
      }
      if (placeholderResId == 0) {
        throw new IllegalArgumentException("Placeholder image resource invalid.");
      }
      if (placeholderDrawable != null) {
        throw new IllegalStateException("Placeholder image already set.");
      }
      this.placeholderResId = placeholderResId;
      return this;
    }