Fix guess_school_id migration

CHANGELOG.rst

@@ -9,6 +9,12 @@ and this project adheres to `Semantic Versioning`_.
 Unreleased
 ----------
 
+Fixed
+~~~~~
+
+* Guessing school ID could fail in `aleksis-admin migrate` under some
+  version conditions
+
 `2.3.1`_ - 2022-08-13
 ---------------------
 

aleksis/apps/untis/migrations/0003_guess_school_id.py

@@ -13,9 +13,10 @@ def guess_school_id(apps, schema_editor):
     except ValidityRange.DoesNotExist:
         return
 
-    school_id = vr.school_id_untis
-    if school_id:
-        get_site_preferences()["untis_mysql__school_id"] = school_id
+    if not vr or not vr.school_id_untis:
+        return
+
+    get_site_preferences()["untis_mysql__school_id"] = school_id
 
 
 class Migration(migrations.Migration):