TreeData addItems causes NPE because it doesn't check childItems

The TreeData.addItems() method below can cause a NullPointerException if any item in the tree has no children.

   public TreeData<T> addItems(Collection<T> rootItems,
            ValueProvider<T, Collection<T>> childItemProvider) {
        rootItems.forEach(item -> {
            addItem(null, item);
            Collection<T> childItems = childItemProvider.apply(item);
            addItems(item, childItems);
            addItemsRecursively(childItems, childItemProvider);
        });
        return this;
    }

The code should check "childItems" to make sure its not null prior to calling addItems(item, childItems). e.g.

   public TreeData<T> addItems(Collection<T> rootItems,
            ValueProvider<T, Collection<T>> childItemProvider) {
        rootItems.forEach(item -> {
            addItem(null, item);
            Collection<T> childItems = childItemProvider.apply(item);
            if (childItems != null) {
                addItems(item, childItems);
                addItemsRecursively(childItems, childItemProvider);
            }
        });
        return this;
    }

My calling code:

TreeData<UserGroupRow> data = new TreeData<>();
if (groups != null)
	data.addItems(groups, UserGroupRow::getUserGroupChildren);

In this code some values returned by getUserGroupChildren may be null.

Vaadin Framework 8.63

Start at the TreeData.addItems() entry point shown in the issue and trace how a null result from the childItemProvider is handled. Reproduce the case with an item whose children are null, then verify that traversal no longer throws a NullPointerException and that non-null child collections are still processed.