It’s only a bug if there are software requirements. Anywho, you can add this line to prevent the code running before it has something to work with (i.e. attempting to populate with nothing):
210108_PopulateValueList_01.gh (4.2 KB)
And these bits to avoid the item selection error raised when populating an empty value list:
210108_PopulateValueList_02.gh (4.4 KB)
It’s getting quite complicated. So I’ll leave to you to add all the use-cases you can think off.