In org.zkoss.zk.ui.HtmlShadowElement.setShadowHost(Component, Component) - line 241
host.getDesktop().getWebApp().getConfiguration().afterShadowAttached(this, host)
will throw a NPE if the component host have a parent, but is not attached to a page.

This may happen when using createComponents or createComponentDirectly to generate components based on a zul fragment without entering a page.

A more informative exception could be something like:
[element] Shadow Element parent cannot be detached

see the attached example.