docs: public void addCustomComponent(Component component) {
  		validateNotTabbed();
  		if (this.fieldList.contains(component)) {
  			throw new IllegalArgumentException("Component already added: " + component);
  		}
  		this.getMainPanel().add(component, 
  				LayoutHelper.getGBC(0, this.fieldList.size(), 2, fieldWeight, 0.0D, GridBagConstraints.BOTH, new Insets(4,4,4,4)));
  		if (this.fieldList.size() == 0) {
  			// First field, always grab focus
  			component.requestFocusInWindow();
  		}
  		this.fieldList.add(component);
  	}