I tried your suggestion but it did not help. I now get the error message because the space does not have the child. In the following method myFrame is an instance variable I keep. Before adding a new object I would like to delete the old one (thats why I keep the instance var), yet it does not work ...
| gTransform p |
myFrame ifNotNil: [
p := harness activeSpace future hasChild: myFrame.
p value ifFalse: [ self error: 'Something is horribly broken'. ]
" Figure out position. "
gTransform := harness avatar globalTransform.