Some further experimentation :
1)Add a morph to the world.
2) open a viewer, (with the "viewersInFlaps" option on)
3) Open the halo for that viewer 4) Drag the viewer to the world (off the flap)
5) Select the morph again, open it's halo, (try) to open a viewer for it (with the other viewer still displayed) and the "undefinedObject>>ShowFlap error is displayed.
If the "dragged/disconnected (from the flap)" viewer is first deleted, the error message doesn't appear.
Mike