When switching to the QtQuick Designer view, I get this console output:

":/componentsplugin/images/scrollbar16.png" does not exist
":/componentsplugin/images/scrollbarh.png" does not exist
":/componentsplugin/images/scrollbarv.png" does not exist

And in fact, these image files are defined in src/plugins/qmldesigner/componentsplugin/components.metainfo but do not exist.

I suggest either using window(16).png as already done for other items without icons or just remove scrollbar from components.metainfo, since ScrollBar does not get shown in the ItemLibrary anyways.