fix(timetable): initialize default objects and arrays if undefined

Author: zwliew

website/src/reducers/timetables.ts

@@ -289,8 +289,11 @@ function timetables(
         draft.lessons[semester] = semTimetable(draft.lessons[semester], action);
         draft.colors[semester] = semColors(state.colors[semester], action);
         draft.hidden[semester] = semHiddenModules(state.hidden[semester], action);
+        if (!draft.customisedModules) {
+          draft.customisedModules = {};
+        }
         draft.customisedModules[semester] = customisedModules(
-          state.customisedModules[semester],
+          state.customisedModules?.[semester],
           action,
         );
       });

website/src/views/timetable/TimetableContent.tsx

@@ -517,7 +517,7 @@ function mapStateToProps(state: StoreState, ownProps: OwnProps) {
     modules,
     activeLesson: state.app.activeLesson,
     customiseModule: state.app.customiseModule,
-    customisedModules: state.timetables.customisedModules[semester],
+    customisedModules: state.timetables.customisedModules?.[semester] ?? [],
     timetableOrientation: state.theme.timetableOrientation,
     showTitle: state.theme.showTitle,
     hiddenInTimetable,