doc link : https://doc.qt.io/QtSafeRenderer/qtsr-qml-types.html#mandatory-properties-for-safe-qml-types

The Note says:

Note: In Qt Design Studio, you can set the fillColor to a color that has full or partial transparency. This results in assert in the qtsafelayouttool as fillColor should have fully opaque color.

This raises a question of "What about Qt Designer and Qt Creator which are also claimed to be usable with Qt Safe Renderer?"
For readers, it's not clear of this is just the writer forgot to mention these tools or they are intentionally left out.