Fix updating subjects

aleksis/apps/cursus/frontend/components/Subject.vue

@@ -212,7 +212,7 @@ export default {
         if (header.value === "parent") {
           dto["parent"] = item.parent?.id;
         } else if (header.value === "teachers") {
-          dto["teachers"] = item.teachers.map((teacher) => teacher.id);
+          dto["teachers"] = item.teachers?.map((teacher) => teacher.id);
         } else {
           dto[header.value] = item[header.value];
         }