QML errors in toolbar editor
Some controls naturally refer to objects that are inaccessible from the toolbar editor dialog. This behavior is unexpected and causes QML errors.
However, all controls are automatically disabled upon populating the views, so these errors do not really affect the usability unless the visualization of the control depends on the inaccessible object.
The proper way of handling this issue in the long term might be creating a proxy control just for the sake of visualization and drag & drop as it makes no sense to instantiate controls in their full-fledged forms in the editor.