[Unhandled Error] Cannot read properties of undefined (reading 'element')

[vs-code-engineering]

Error Message

Cannot read properties of undefined (reading 'element')

Stack Trace

    at hR.element (./src/vs/base/browser/ui/list/listView.ts:845:27)
    at (anonymous) (./src/vs/base/browser/ui/list/listWidget.ts:240:97)
    at i4e.splice (./src/vs/base/browser/ui/list/listWidget.ts:240:49)
    at (anonymous) (./src/vs/base/browser/ui/list/splice.ts:17:34)
    at Lyt.splice (./src/vs/base/browser/ui/list/splice.ts:17:19)
    at (anonymous) (./src/vs/base/browser/ui/list/listWidget.ts:1621:56)
    at fn (./src/vs/base/common/event.ts:1828:12)
    at Hl.splice (./src/vs/base/browser/ui/list/listWidget.ts:1621:21)
    at ZWe.filterItems (./src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts:1824:0)
    at ZWe.loadItems (./src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts:1171:0)
    at async ZWe.refresh (./src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts:1155:0)

Raw Stack Trace (minified)

TypeError: Cannot read properties of undefined (reading 'element')     at hR.element (out/vs/workbench/workbench.desktop.main.js:685:19795)     at out/vs/workbench/workbench.desktop.main.js:685:39456     at Array.map (<anonymous>)     at i4e.splice (out/vs/workbench/workbench.desktop.main.js:685:39411)     at out/vs/workbench/workbench.desktop.main.js:683:125     at Array.forEach (<anonymous>)     at Lyt.splice (out/vs/workbench/workbench.desktop.main.js:683:112)     at out/vs/workbench/workbench.desktop.main.js:728:8948     at eO.bufferEvents (out/vs/workbench/workbench.desktop.main.js:405:6915)     at Hl.splice (out/vs/workbench/workbench.desktop.main.js:728:8915)     at ZWe.filterItems (out/vs/workbench/workbench.desktop.main.js:3390:23835)     at ZWe.loadItems (out/vs/workbench/workbench.desktop.main.js:3390:13631)     at async ZWe.refresh (out/vs/workbench/workbench.desktop.main.js:3390:13457)

Details

Property	Value
Bucket ID	7835c5b7-a3ac-eeee-ed21-038c35d8dbd1
Version	1.115.0-insider
Commit	69eb4623
Last Seen	Thu Apr 02 2026 23:43:13 GMT+0000 (Coordinated Universal Time)
Platforms	Windows, Mac, Linux
Product	VSCode

---

This issue was automatically created from the VS Code Errors Dashboard

[vs-code-engineering]

Regression Details

• Regression type: new
• Last release without this error: 1.114.0-insider
• First release with this error: 1.115.0-insider
• Commit range: ee6bfc55...dcd017c9

Analysis

This error occurs in listView.ts:element when aiCustomizationListWidget.ts:filterItems → loadItems → refresh triggers a list splice operation. The list view tries to access an element at an index that no longer exists, likely due to a race condition where the list data changes between the filter computation and the splice application.

Note: A similar "Cannot read properties of undefined (reading 'element')" error exists under #212229, but that issue tracks a different codepath through notebook cell lists. This bucket represents a distinct trigger path through the AI customization list widget.

Key source files: aiCustomizationListWidget.ts, listView.ts, listWidget.ts, splice.ts

Generated by errors-regression-scan · ◷

[vs-code-engineering]

🔍 Error Investigation

Summary

The error Cannot read properties of undefined (reading 'element') occurs in the TraitSpliceable.splice() method when the AI customization list widget calls list.splice() during a refresh. The focus/selection trait holds a stale index that exceeds the view's items array length, causing a crash when the trait tries to read the old element identity. This is a race condition triggered by multiple concurrent loadItems() calls from overlapping autoruns.

Culprit Commit

No single commit in the regression range (ee6bfc55...dcd017c9) modified aiCustomizationListWidget.ts or the base list widget files — the file SHA is identical at both endpoints. The error was exposed by sessions sidebar lifecycle changes in this range (commits 3bc76baebe, 29f5047784, and related sidebar/sessions commits by @mrleemurray and @sandy081) that changed when the customizations panel is rendered, increasing the likelihood of concurrent autorun triggers.

The underlying bug was a pre-existing race condition in loadItems() — multiple concurrent async calls from overlapping autoruns could resolve out of order, overwriting allItems with stale/empty data. This was confirmed and already fixed in PR #307745 by @joshspicer (merged Apr 6).

Code Flow

sequenceDiagram
    participant Autorun as Autoruns (×4+)
    participant Widget as AICustomizationListWidget
    participant LoadItems as loadItems() [async]
    participant FilterItems as filterItems()
    participant List as List.splice()
    participant Trait as TraitSpliceable
    participant View as ListView

    Autorun->>Widget: refresh() (concurrent triggers)
    Widget->>LoadItems: loadItems() call A (seq=1)
    Autorun->>Widget: refresh() (second trigger)
    Widget->>LoadItems: loadItems() call B (seq=2)
    Note over LoadItems: ⚠️ Race condition:<br/>Call A resolves after B,<br/>overwrites allItems with<br/>stale/empty data
    LoadItems->>FilterItems: filterItems()
    FilterItems->>List: splice(0, length, newEntries)
    List->>Trait: TraitSpliceable.splice()
    Trait->>View: view.element(staleIndex)
    Note over View: 💥 this.items[index] is<br/>undefined — index out of bounds

Loading

Affected Files

File	Role
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts	Root cause — race condition in loadItems() / filterItems() / commitDisplayEntries()
src/vs/base/browser/ui/list/listWidget.ts:240	Crash site — TraitSpliceable.splice() reads view element at stale trait index
src/vs/base/browser/ui/list/listView.ts:845	Crash site — element(index) accesses this.items[index].element on undefined
src/vs/base/browser/ui/list/splice.ts:17	Data flow — CombinedSpliceable iterates trait spliceables before view

Repro Steps

1. Enable Insiders build (1.115.0) with the AI customization panel visible
2. Open the customizations view in the sessions sidebar
3. Trigger rapid state changes that fire multiple autoruns simultaneously — e.g., switching harnesses, registering providers, or changing workspaces while the panel is loading
4. The race condition is non-deterministic; it requires two loadItems() calls to overlap such that the slower one resolves after the faster one and overwrites allItems with stale data

Fix Proposal

Already fixed in PR #307745 (merged Apr 6, 2026).

The fix adds a _loadItemsSeq sequence counter to loadItems() so that only the latest async call's result is applied. It also fixes missing onDidChangeInstructions subscriptions, re-establishes provider onDidChange autorun when harnesses register, and adds instruction-semantic groups to filterItemsForProvider to prevent items from being silently dropped.

Fix location: src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts → loadItems()
Approach: Sequence counter discards stale async results; additional subscription fixes prevent missed refresh triggers.

Recommended Owner

@joshspicer — authored the fix in PR #307745 and is actively working on the AI customization provider API.

Generated by errors-investigate · ◷

[joshspicer]

super cool! Yes I beleive both the bug and the analyzed fix is correct!

[vs-code-engineering]

Regression Note

This error continues to appear in 1.115.0-insider (3d70aab9) with a new bucket ID ecaba6eb, affecting 120 users (198 hits) across Windows, Mac, and Linux.

The deminified callstack is identical to the original — listView.ts → listWidget.ts → splice.ts → aiCustomizationListWidget.ts (commitDisplayEntries → filterItemsForCore → filterItems → loadItems → refresh).

The bucket ID changed due to minification drift between builds. Linked new bucket to this issue.

Generated by errors-regression-scan · ◷

[vs-code-engineering]

Regression Detected (Agents)

This error has reappeared in the Agents product on 1.116.0-insider (fc6d18c3) with a new bucket ID.

• Product: Agents
• New bucket ID: 4602694c
• Impact: 29 hits, 22 users
• Platforms: Windows, Mac
• Deminified stack: listView.ts → listWidget.ts → splice.ts → aiCustomizationListWidget.ts (commitDisplayEntries → filterItemsForCore → filterItems → loadItems → refresh)
• Commit range: 41dd792b...fc6d18c3

The stack is nearly identical to the original error — the fix may not have fully resolved the issue, or it has regressed in this new build.

Generated by errors-agents-regression-scan · ◷

[vs-code-engineering]

Regression Details

This error has reappeared in 1.117.0-insider on the Agents product despite the fix in milestone 1.116.0.

• Product: Agents
• Regression type: New error (re-regression)
• Bucket ID: 6fe628e1
• Users: 22 (30 hits)
• Platforms: Windows, Mac
• Last release without this error: 1.116.0-insider (ae130017)
• First release with this error: 1.117.0-insider (98b4d3e2)
• Commit range: ae130017...98b4d3e2

Deminified Stack (1.117.0-insider)

at (anonymous) (listView.ts:845:27)
at (anonymous) (listWidget.ts:240:97)
at (anonymous) (listWidget.ts:240:49)
at (anonymous) (splice.ts:17:34)
at (anonymous) (splice.ts:17:19)
at (anonymous) (listWidget.ts:1621:56)
at fn (event.ts:1829:12)
at (anonymous) (listWidget.ts:1621:21)
at (anonymous) (aiCustomizationListWidget.ts:1929:0) → commitDisplayEntries
at (anonymous) (aiCustomizationListWidget.ts:2050:0) → filterItemsForCore
at (anonymous) (aiCustomizationListWidget.ts:2063:0) → filterItems
at (anonymous) (aiCustomizationListWidget.ts:1190:0) → loadItems
at (anonymous) (aiCustomizationListWidget.ts:1165:0) → refresh

The stack is identical to the original issue but now includes additional commitDisplayEntries and filterItemsForCore frames, suggesting the aiCustomizationListWidget code has been refactored since the original fix. The fix from 1.116.0 may not have fully addressed the race condition or a new code path reintroduced it.

Generated by errors-agents-regression-scan · ● 5.7M · ◷

[vs-code-engineering]

Related Error Buckets

This error has recurred on v1.117.0-insider (Agents product) with a new bucket ID:

Bucket	Error Message	Version
6fe628e1	Cannot read properties of undefined (reading 'element')	1.117.0-insider

30 hits, 22 users on 1.117.0-insider, all platforms. Same call path: listView.element → listWidget.splice → aiCustomizationListWidget.commitDisplayEntries → filterItemsForCore → filterItems → loadItems → refresh. The fix from milestone 1.116.0 may not fully cover this code path in the Agents/sessions bundle.

Generated by errors-agents-triage · ● 7.5M · ◷