public void update() {
   LayerTreeTreeNode selected = tree.getSelectedLayerTreeNode();
   if (selected != null && action.isEnabled(selected.getLayer())) {
     setDisabled(false);
     setIcon(action.getIcon());
     setTooltip(action.getTooltip());
   } else {
     setDisabled(true);
     setIcon(action.getDisabledIcon());
     setTooltip("");
   }
 }