code: public static RowSpec createGap(ConstantSize gapHeight) {
          return new RowSpec(RowSpec.DEFAULT, gapHeight, FormSpec.NO_GROW);
      }
docs: Creates and returns a {@link RowSpec} that represents a gap with the specified
  {@link ConstantSize}.
  
  @param gapHeight specifies the gap height
  @return a RowSpec that describes a vertical gap with the given height
  
  @throws NullPointerException if {@code gapHeight} is {@code null}
  
  @since 1.2