Added QML type based on QtObject hangs QML Viewer if source file is open in editor.

I am adding a type using QML, where my new type extends QtObject:

--begin Person.qml--
import Qt 4.7

QtObject {
property string name;
property int shoeSize;
}
--end Person.qml--

I've opened in Qt Creator the (trivial) test UI project that uses this and am running it by clicking the green triangle 'run' button. When no editors are open, it runs fine. But when the above file Person.qml is open in an editor pane, then when I try to run my UI, QML Viewer reports: "QDeclarativeView only supports loading of root objects that derive from QGraphicsObject" and must be forced to quit.

Using the latest internal SDK, updated today. "About Qt Creator" reports "Built on Feb 7 2011 at 11:01:55" and "From revision 486dcd8fd8".