Fix checking related tasks length

ldvspec/lofardata/tasks.py

@@ -169,7 +169,7 @@ def insert_task_into_atdb(workspecification_id: int):
     atdb_predecessor_task_id: int | None = None
     if work_spec.predecessor_specification is not None:
         predecessor: WorkSpecification = work_spec.predecessor_specification
-        if len(predecessor.related_tasks != 1):
+        if len(predecessor.related_tasks) != 1:
             logger.error("Workspecification {} has no valid predecessor".format(work_spec.pk))
             raise InvalidPredecessor()
         # Should only be 1 entry