Using sticky within a Hops component

I am trying to implement the following solution, which uses sticky and updates the receiver as the sender changes values, within a Hops component. When I execute the hops component I do not get a result from the process that requires the feedback, instead, I receive the following warning messages (answer_stk is the sticky variable):

1. Solution exception:'NoneType' object has no attribute 'Document': component "GhPython Script" (4e337e87-bc74-48bf-9f20-5c4b942f2e10)
2. Solution exception:answer_stk: component "GhPython Script" (aac58204-c481-4e76-96df-1a71213774f1)

Any ideas on how this might be solved?
I intend to use this method in a large project, but it would be hard to explain, so here is a dummy test file with simple mathematical operations but the same warning messages: (15.5 KB)
Thanks in advance!