code: public void addLoadedExtension(Extension extension) {
  		if (extension == null) {
  			throw new IllegalArgumentException("Parameter extension must not be null.");
  		}
  
  		if (loadedExtensions == null) {
  			loadedExtensions = new ArrayList<>(1);
  		}
  
  		if (!loadedExtensions.contains(extension)) {
  			loadedExtensions.add(extension);
  			extension.setAddOn(this);
  		}
  	}
docs: Adds the given {@code extension} to the list of loaded extensions of this add-on.
  <p>
  This add-on is set to the given {@code extension}.
  
  @param extension the extension of this add-on that was loaded
  @throws IllegalArgumentException if extension is {@code null}
  @since 2.4.0
  @see #removeLoadedExtension(Extension)
  @see Extension#setAddOn(AddOn)