antonisa-proto Is it a Sketch bug or is something that we always need to check?
Not a bug, it's just how symbol master lookup works (now): it relies on a
parentObject to acquire its document reference — thus the requirement.
antonisa-proto Also, is there any workaround to get the symbol instance sub-layers if the parent object is null?
It depends on how you detach symbol instances. Can you share your code here? One caveat that I'm aware of is that as soon as your original symbol instance is detached, its
parentObject reference is gone (as one would expect). You may need to restore this reference to be able to call
symbolMaster on this instance later:
let instance = <...>
let detached = instance.detach()
instance.parentObject = detached.parentObject()