`ContainerNode.getNode(int)` too forgiving

It would be nice if `ContainerNode.getNode(int)` could accept only valid indices. The current implementation hides some potential bugs. 

```java
    public Node getNode(int index) {
        if (index >= 0 && index < nodes.size()) {
            return nodes.get(index);
        }
        return null;
    }
```