NPE when instantiating an unsolvable problem

[o-fir]

Getting a null pointer exception when trying to instantiate a problem that's clearly not satisfiable

Exception in thread "main" java.lang.NullPointerException
        at fr.uga.pddl4j.problem.FinalizedProblem.finalizeCondition(FinalizedProblem.java:825)
        at fr.uga.pddl4j.problem.FinalizedProblem.finalizeCondition(FinalizedProblem.java:832)
        at fr.uga.pddl4j.problem.FinalizedProblem.finalizeGoal(FinalizedProblem.java:781)
        at fr.uga.pddl4j.problem.HTNProblem.finalization(HTNProblem.java:224)
        at fr.uga.pddl4j.problem.AbstractProblem.instantiate(AbstractProblem.java:438)

If my goal state contains (task_performed task1), but I don't declare an (is_child task1 SOME_ACTIVITY) predicate, which is required to perform the task, I get a NPE.

To reproduce this behaviour, you can use the domain and problem below. Adding (is_child task1 a_no_children) (is_child task2 a_no_children) to the initial state fixes the problem.

My domain:

(define (domain tmp)

(:requirements :hierarchy
	  :typing
	  :method-preconditions
  :negative-preconditions
)

(:types study activity task resource place timeslot)


(:predicates (activity_has_tasks ?a - activity) (activity_primitive ?a - activity) (is_child ?t - task ?a - activity) (task_performed ?t - task)(activity_performed ?a - activity)

)

(:task do_tasks
  :parameters (?a - activity))

; m0_do_tasks is a recursive method that will try to perform some existing task for activity a, and then call itself again
; we need this recursivity because domain doesn't know how many tasks are there in an activity
(:method m0_do_tasks
  :parameters (?a - activity ?t - task ?s - timeslot)
  :task (do_tasks ?a)
  :precondition (and (is_child ?t ?a))
  :ordered-subtasks(and (t1 (schedule_task ?t ?s)) (t2 (do_tasks ?a)) )
)
; m1_do_tasks serves as termination for the recursive m0_do_tasks
(:method m1_do_tasks
  :parameters (?a - activity)
  :task (do_tasks ?a)
  :ordered-subtasks(and (t1 (nop)))
)

(:action schedule_task
  :parameters (?t - task ?s - timeslot)
  :precondition (and (not(task_performed ?t)))
  :effect(and (task_performed ?t))
)

(:action nop
  :parameters ()
  :precondition ()
  :effect ()
)
)

My problem:

(define (problem test)
(:domain tmp)

;---------------- Facts -----------------------
(:objects
a_no_children a_w_children - activity
task1 task2 task3 task4 - task
t1 t2 t3 t4 t5 - timeslot
bob anna - resource
)

;--------------- Initial State -----------------
	(:htn
		:parameters ()
		:subtasks (and
		 (task0 (do_tasks a_no_children))
		)
	)
(:init
    
)

(:goal
    (and (task_performed task1)
    (task_performed task2)
    )
)

)