router.mountDevtools(el) needs a router with metrics()/on() and a DOM element to render into; the panel was not mounted.

Warnings reference › data-router:devtools-target

Identifier

data-router:devtools-target

Raised by

Data Router

Since

1.34.0

What happened

The observability panel had either no usable router or no element to draw into. A no-op controller is returned - its destroy() and refresh() do nothing - so calling code is safe, but there is no panel.

The fix

Pass the router and a real element. In a headless environment the panel cannot be mounted at all.

The smallest fix

router.mountDevtools(document.querySelector('#router-devtools'));

Reference

Covered in full at the API reference.