Fix the bug that occurs when setting the factor for a grade subject that doesn't yet exist.

app/lib/grades/pages/term_settings_page/term_settings_page_controller.dart

@@ -148,10 +148,11 @@ class TermSettingsPageController extends ChangeNotifier {
   }
 
   Future<void> setSubjectWeight(SubjectId subjectId, Weight weight) async {
-    final subRef = termRef.subject(subjectId);
+    TermSubjectRef subRef = termRef.subject(subjectId);
     final isNewSubject = gradesService.getSubject(subjectId) == null;
     if (isNewSubject) {
-      subjectId = _createSubject(subRef);
+      final newSubjectId = _createSubject(subRef);
+      subRef = termRef.subject(newSubjectId);
 
       // Firestore had a soft limit of 1 write per second per document. However,
       // this limit isn't mentioned in the documentation anymore. We still keep

app/test/grades/pages/term_settings_page/term_settings_page_controller_test.dart

@@ -0,0 +1,56 @@
+// Copyright (c) 2026 Sharezone UG (haftungsbeschränkt)
+// Licensed under the EUPL-1.2-or-later.
+//
+// You may obtain a copy of the Licence at:
+// https://joinup.ec.europa.eu/software/page/eupl
+//
+// SPDX-License-Identifier: EUPL-1.2
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:group_domain_models/group_domain_models.dart';
+import 'package:sharezone/grades/grades_service/grades_service.dart';
+import 'package:sharezone/grades/pages/term_settings_page/term_settings_page_controller.dart';
+
+import '../../grades_test_common.dart';
+
+void main() {
+  group('$TermSettingsPageController', () {
+    test(
+      'setSubjectWeight creates a missing subject and updates the weight',
+      () async {
+        const termId = TermId('term-id');
+        const courseId = 'course-id';
+        final service = GradesService();
+        final testController = GradesTestController(gradesService: service);
+        testController.createTerm(termWith(id: termId));
+
+        final controller = TermSettingsPageController(
+          gradesService: service,
+          termRef: service.term(termId),
+          coursesStream: Stream.value([
+            Course.create().copyWith(
+              id: courseId,
+              name: 'English LK',
+              subject: 'English',
+              abbreviation: 'EN',
+            ),
+          ]),
+        );
+
+        await Future<void>.delayed(Duration.zero);
+        await controller.setSubjectWeight(
+          const SubjectId(courseId),
+          const Weight.factor(2),
+        );
+
+        final termSubject = testController
+            .term(termId)
+            .subjects
+            .singleWhere((subject) => subject.name == 'English');
+        expect(termSubject.weightingForTermGrade, const Weight.factor(2));
+
+        controller.dispose();
+      },
+    );
+  });
+}