code: @Nullable
      public Element getElement(@Nonnull String id) {
          if (id == null || id.trim().length() == 0) {
              throw new IllegalArgumentException("An element ID must be specified.");
          }
  
          return elementsById.get(id);
      }
docs: Gets the element with the specified ID.
  
  @param id the {@link Element#getId()} of the element
  @return the element in this model with the specified ID (or null if it doesn't exist)
  @see Element#getId()