I have a jenkins workflow/pipeline job in which i use the Active choices plugin in parent child mode
This pipeline job is the downstream job for another job
My script is attached
create a sample pipeline job with the attached script(Job1)
Create pipeline Job2->Snippet Generator>build:Build a Job->Give Job1, Observe that CHILD field throws an exception and goes to fall back script

Script is attached(active_choice_issue.groovy)
and error is attached
Chrome being the browser(latest edition on windows 10)