I think the problem is only that restnodes isn't properly loaded when called. It's either null or empty, which means there are no nodes in the group
"nodes", in which case you should add a if statement to check if restnodes contains elements :
if rest_nodes.size() < 0):
Are you trying to get the position of an object? There is maybe an easier way to do so that doesn't require groups. If you say more about it, I could try to help you find a better replacement.