Fix error with manual assessment "next" link

Fixed error when previous submitter was not found.

exercise/staff_views.py

@@ -377,24 +377,25 @@ def get(self, request: HttpRequest, *args: Any, **kwargs: Any) -> HttpResponse:
         total_submitters = submitters.count()
         previous_user_id = request.GET.get('prev')
         if previous_user_id:
-            # get the previous time
-            previous_time = submitters.filter(user__id=previous_user_id).first().earliest_submission
-
-            # remove the submitters who have been assessed (we do this after we've found the
-            # previous submitter's time as the previous submitter might have been assessed)
-            submitters = submitters.filter(count_assessed=0)
-
-            # Find specifically the submitter who's submission was submitted right after this one
-            submitters = submitters.filter(earliest_submission__gt=previous_time)
-            submitter = submitters.first()
+            previous_submitter = submitters.filter(user__id=previous_user_id).first()
+            if previous_submitter:
+                # get the previous time
+                previous_time = previous_submitter.earliest_submission
+                if previous_time:
+                    # remove the submitters who have been assessed (we do this after we've found the
+                    # previous submitter's time as the previous submitter might have been assessed)
+                    submitters = submitters.filter(count_assessed=0)
+
+                    # Find specifically the submitter who's submission was submitted right after this one
+                    submitters = submitters.filter(earliest_submission__gt=previous_time)
+                    submitter = submitters.first()
 
         if not submitter:
             submitters = submitters.filter(count_assessed=0)
             submitter = submitters.first()
 
-        # the number of submitters that we have after filtering
-        submitters_after = submitters.count()
-        filtered_submitters = total_submitters - submitters_after + 1
+        # Subtract the number of submitters that we have after filtering from the total_submitters
+        filtered_submitters = total_submitters - submitters.count() + 1
 
         if not submitter:
             # There are no more unassessed submitters.
@@ -406,7 +407,10 @@ def get(self, request: HttpRequest, *args: Any, **kwargs: Any) -> HttpResponse:
         ids = cache.submission_ids(exercise_id=self.exercise.id, best=True, fallback_to_last=True)
         if not ids:
             raise Http404()
-        percentage = f"{int(filtered_submitters / total_submitters * 100)}%"
+        if total_submitters == 0:
+            percentage = "0%"
+        else:
+            percentage = f"{int(filtered_submitters / total_submitters * 100)}%"
         self.request.session['manually_assessed_counter'] = (
             f"{filtered_submitters} / {total_submitters} ({percentage})"
         )