Unable to delete a PageNode that has child nodes

Bug description

If you attempt to delete a PageNode that contains one or more child nodes, the XUI fall over with an exception.

How to reproduce the issue

1. Create a authentication tree with a page node and add one or more child nodes.
2. Now try to delete the page node.

Expected behaviour

It is removed from the tree, along with its child nodes.

Current behaviour

Boom! The UI falls over (blank screen), nothing is deleted.