public void createNodes(QuizCatalogus quizCatalogus){
		top.removeAllChildren();
		try {
			DefaultMutableTreeNode quiz = null;
			DefaultMutableTreeNode opdracht = null;
			List<Quiz> quizzen = quizCatalogus.getQuizzen();
			for(Quiz q : quizzen){
				String nameQuiz = q.toString();
		
				quiz = new DefaultMutableTreeNode(nameQuiz);
				top.add(quiz);
		
				for (Opdracht opdr : q.getOpdrachten()) {
					String nameOpdracht = opdr.toString();
					opdracht = new DefaultMutableTreeNode(nameOpdracht);
					quiz.add(opdracht);
				}
			}
			
			DefaultMutableTreeNode currentNode = top.getNextNode();
		    do {
		       if (currentNode.getLevel()==1) 
		            tree.expandPath(new TreePath(currentNode.getPath()));
		       currentNode = currentNode.getNextNode();
		       }
		    while (currentNode != null);
			
		} catch (Exception e) {
			System.out.println(e.getMessage());
		}
	}