Code:
	public static void main(final String[] args) {

		final BaseTreeModel model1 = new BaseTreeModel();
		model1.add(new BaseTreeModel());
		model1.add(new BaseTreeModel());
		model1.add(new BaseTreeModel());
		model1.add(new BaseTreeModel());
		
		List<ModelData> children = model1.getChildren();

		final BaseTreeModel model2 = new BaseTreeModel();
		model2.setChildren(children);

		System.out.println(model2.getChildCount());

	}
HTML Code:
Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.AbstractList$Itr.checkForComodification(Unknown Source)
	at java.util.AbstractList$Itr.next(Unknown Source)
	at com.extjs.gxt.ui.client.data.BaseTreeModel.setChildren(BaseTreeModel.java:178)