UList does not render...

[sherpal]

... unless some other component such as SideNavigation is used.

This is a known issue for version up to 1.8.1, and it will be fixed in 1.9.0. If you are below 1.9.0 and don't want to/can't change, you can import manually this (in your index.html, before importing your scala-js code):

<script type="module">import "@ui5/webcomponents/dist/StandardListItem.js"</script>

A similar issue might appear if you use group header list item, for which you need this import: import @ui5/webcomponents/dist/GroupHeaderListItem.js.