some component properties have overloaded signatures
e.g.XulElement.setContext(String popupId) / setContext(Popup popup)
setPopup setTooltip ... maybe more

In ZK 8 it is no longer possible to assign a popup directly using EL or ZK-Bind annotations

see attachment for reproducing examples.

WORKAROUND:
use the popup-Id instead

<popup id="mypopup"/>
<label context="${mypopup}"/> <!-- FAILS -->
<label context="${mypopup.id}"/> <!-- WORKS -->
<label context="mypopup"/> <!-- even simpler no EL just the id -->