add none check

Author: SKairinos

codeforlife/user/models/teacher.py

@@ -175,6 +175,10 @@ def teacher_as_type(
     Returns:
         An instance of the typed teacher.
     """
+    # TODO: remove in new schema
+    if teacher is None:
+        return None
+
     return typed_teacher_class(
         pk=teacher.pk,
         user=teacher.user,