threaded adherence cache production error (#4332)

add guard and premature exit should adherence cache be working on
timeline data purged in another thread.

portal/models/qb_status.py

@@ -3,6 +3,7 @@
 API to lookup user's status with respect to assigned questionnaire banks.
 
 """
+from flask import current_app
 from ..trace import trace
 from .overall_status import OverallStatus
 from .qb_timeline import QBT, ordered_qbs, update_users_QBT
@@ -154,7 +155,6 @@ def _status_from_current(self, cur_qbd):
         # Withdrawn sanity check
         if self.withdrawn_by(self.as_of_date) and (
                 self.overall_status != OverallStatus.withdrawn):
-            from flask import current_app
             current_app.logger.error(
                 "Unexpected state %s, user %d should be withdrawn",
                 self.overall_status, self.user.id)
@@ -187,6 +187,13 @@ def older_qbds(self, last_known):
                 QBT.qb_iteration == cur_qbd.iteration).order_by(
                 QBT.at.desc(), QBT.id.desc()).with_entities(
                 QBT.status).first()
+            # production errors seen, where qb_timeline loses data.  likely
+            # a race condition where adherence cache is using timeline rows
+            # when another thread purges the user's timeline.  log and exit
+            if status is None:
+                current_app.logger.info(
+                    f"timeline data disappeared mid loop {self.user.id}: {QBT}")
+                return
             yield self.__ordered_qbs[index], str(status[0])
 
     def _indef_init(self):
@@ -475,7 +482,6 @@ def warn_on_duplicate_request(self, requested_set):
 
         Once bug is found and resolved, remove!
         """
-        from flask import current_app
         from .questionnaire_response import QuestionnaireResponse
 
         requested = requested_set.intersection(('irondemog', 'irondemog_v3'))