Fix crash when stored playground language state is invalid (#3646)

* Fix crash when stored playground language state is invalid

Validate the stored chapter+variant combination against ALL_LANGUAGES
when loading from localStorage. Falls back to the default language config
if the combination is invalid (e.g. chapter 1 + explicit-control).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Fall back to defaultLanguageConfig for invalid stored language state

When the stored chapter+variant combination is not in ALL_LANGUAGES,
reset to `defaultLanguageConfig` (a known-valid pair) rather than to the
individual default context fields, so the fallback is explicitly "the
default language config" as the issue describes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Richard Dominick <34370238+RichDom2185@users.noreply.github.com>

Author: 3 people

src/pages/createStore.ts

@@ -4,7 +4,12 @@ import { throttle } from 'lodash-es';
 import createSagaMiddleware from 'redux-saga';
 import type { SourceActionType } from 'src/commons/utils/ActionsHelper';
 
-import { defaultState, type OverallState } from '../commons/application/ApplicationTypes';
+import {
+  ALL_LANGUAGES,
+  defaultLanguageConfig,
+  defaultState,
+  type OverallState,
+} from '../commons/application/ApplicationTypes';
 import rootReducer from '../commons/application/reducers/RootReducer';
 import MainSaga from '../commons/sagas/MainSaga';
 import { generateOctokitInstance } from '../commons/utils/GitHubPersistenceHelper';
@@ -79,15 +84,25 @@ function loadStore(loadedStore: SavedState | undefined) {
         externalLibrary: loadedStore.playgroundExternalLibrary
           ? loadedStore.playgroundExternalLibrary
           : defaultState.workspaces.playground.externalLibrary,
-        context: {
-          ...defaultState.workspaces.playground.context,
-          chapter: loadedStore.playgroundSourceChapter
+        context: (() => {
+          const chapter = loadedStore.playgroundSourceChapter
             ? loadedStore.playgroundSourceChapter
-            : defaultState.workspaces.playground.context.chapter,
-          variant: loadedStore.playgroundSourceVariant
+            : defaultState.workspaces.playground.context.chapter;
+          const variant = loadedStore.playgroundSourceVariant
             ? loadedStore.playgroundSourceVariant
-            : defaultState.workspaces.playground.context.variant,
-        },
+            : defaultState.workspaces.playground.context.variant;
+          const validCombination = ALL_LANGUAGES.some(
+            lang => lang.chapter === chapter && lang.variant === variant,
+          );
+          // Invalid stored combinations (e.g. chapter 1 + explicit-control, which can be
+          // persisted when toggling feature flags) would crash on load, so fall back to
+          // the default language config as a coherent, always-valid pair.
+          return {
+            ...defaultState.workspaces.playground.context,
+            chapter: validCombination ? chapter : defaultLanguageConfig.chapter,
+            variant: validCombination ? variant : defaultLanguageConfig.variant,
+          };
+        })(),
       },
     },
   };