Invalid z_index for sketchpad element
An error appeared during simple edition activity on an experiment and I was not able to locate the problem. I am also confused by the 'RuntimeError' statement in the debug window. Could you please orient my debugging ? The experiment does not contain any special or tricky feature, it worked fine just before minor modifications have been made, with no change on the structure itself. Little changes that I don't even remember (cosmetic changes).
Thanks in advance,