code: private HtmlContainer getOrCreateLayerContainer(Layer layer) {
  		if (layerContainers.containsKey(layer)) {
  			return layerContainers.get(layer);
  		}
  		HtmlContainer layerContainer = new HtmlGroup();
  		layerContainer.asWidget().getElement().setId(layer.getId());
  		layersModelContainer.add(layerContainer);
  		layerContainers.put(layer, layerContainer);
  		return layerContainer;
  	}
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