Old method don't make null children ArrayList.
But the first access it is null.
Is better modify to return a null ArrayList when invoking getChildren() methods or is better nullify the children ArrayList in public method detachAllChildren like this?
public void detachAllChildren() {
if(children != null) {
for ( int i = children.size() - 1; i >= 0; i-- ) {
detachChildAt( i );
}
logger.info("All children removed.");
children = null;
}
}
Children is null by default to avoid unnecessary object creation… however, if a Node has had children, it is more likely to have children in the future, so I don't see any reason to force it back to null.
That's why you'd use the provided getQuantity() 
if (node.getChildren() != null)
fail if is called after node.deteachAllChildren(), because the ArrayList is not null.
If you use condition
if (node.getChildren().isEmpty())
"valid context after populating node", fail at first time by nullReferenceException.