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