• Create 3D project
• Add a sphere primitive into the 3D scene
• Right click on sphere in 3D view and select "Add to Content Library"
• Create new 3D project
• Drag the previously added sphere from content library into the 3D view
• Right click the sphere in navigator and select "Edit component"

--> The component's scene in navigator only shows "Item" with warning triangle.

This happens because the created component lacks QtQuick import.