stored in two separate lists
Still don’t know WHERE these lists are stored. Externally? In Python? In Java? In URScript?
“index out of range, the list size is smaller than 0 in UR robot”
This is what you need to include in your original post. Arrays are not dynamic on the UR. You need to initialize them with a size when they are declared.
Other list functions are somewhat recent, so make sure you’re up to at least Polyscope 5.15, otherwise the “append()” method doesn’t even exist.
See the script manual here for more explanation on lists.