If NullList is an inner class of Tools, I don’t think you can access it without accessing Tools first. If you want to access it directly, you need to create a separate script with the class_name NullList. You don’t need to make NullList a singleton.
An example of the first line of NullList script:
class_name NullList extends Node
Now you should be able to access it directly (NullList.new())