fix(web): prevent null folder tree on concurrent load (#26489)

web/src/lib/stores/folders.svelte.spec.ts

@@ -0,0 +1,42 @@
+import { foldersStore } from '$lib/stores/folders.svelte';
+import { getUniqueOriginalPaths } from '@immich/sdk';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('$lib/managers/event-manager.svelte', () => ({
+  eventManager: {
+    on: vi.fn(),
+  },
+}));
+
+vi.mock('@immich/sdk', () => ({
+  getAssetsByOriginalPath: vi.fn(),
+  getUniqueOriginalPaths: vi.fn(),
+}));
+
+describe('foldersStore', () => {
+  beforeEach(() => {
+    foldersStore.clearCache();
+    vi.clearAllMocks();
+  });
+
+  it('returns the same non-null tree for concurrent fetchTree calls', async () => {
+    let resolvePaths: (value: string[]) => void;
+
+    vi.mocked(getUniqueOriginalPaths).mockReturnValue(
+      new Promise<string[]>((resolve) => {
+        resolvePaths = resolve;
+      }),
+    );
+
+    const first = foldersStore.fetchTree();
+    const second = foldersStore.fetchTree();
+
+    resolvePaths!(['/photos/2026']);
+
+    const [firstTree, secondTree] = await Promise.all([first, second]);
+
+    expect(firstTree).not.toBeNull();
+    expect(secondTree).not.toBeNull();
+    expect(secondTree).toEqual(firstTree);
+  });
+});

web/src/lib/stores/folders.svelte.ts

@@ -28,10 +28,9 @@ class FoldersStore {
     if (this.initialized) {
       return this.folders!;
     }
-    this.initialized = true;
-
     this.folders = TreeNode.fromPaths(await getUniqueOriginalPaths());
     this.folders.collapse();
+    this.initialized = true;
     return this.folders;
   }