Fix detection of the end of the validity ranges list in next_lessons

CHANGELOG.rst

@@ -21,6 +21,7 @@ Fixed
   user language (fixed by using gettext_lazy).
 * Affected groups and persons in substitutions list were displayed multiple times.
 * ``lessons_on_day`` didn't work as expected if a person has no lessons.
+* End of validity ranges list wasn't detected correctly in ``next_lesson``.
 
 `2.0b2` - 2021-06-02
 --------------------

aleksis/apps/chronos/managers.py

@@ -465,7 +465,7 @@ class LessonDataQuerySet(models.QuerySet, WeekQuerySetMixin):
         # If date of next lesson period is out of validity range (larger) ...
         elif next_lesson_period_date > next_lesson_period.lesson.validity.date_end:
             # ... we have to get the lesson period from the next validity range
-            if validity_index >= len(validity_ranges):
+            if validity_index >= len(validity_ranges) - 1:
                 # There are no validity ranges (and thus no lessons)
                 # in the school term after this lesson period
                 return None