Merging from the PR description tab shows "Error refreshing pull request description: Webview is disposed" #9143

Bug. After merging a PR from the PR description tab, VS Code shows the error toast Error refreshing pull request description: Webview is disposed. The merge itself, the branch deletion and the checkout all succeed; the tab was going to close anyway, so the toast is noise.

Steps

1. Set "githubPullRequests.deleteBranchAfterMerge": true and "githubPullRequests.postDone": "checkoutDefaultBranchAndPull".
2. Check out a PR branch and open its description tab.
3. Merge the PR from the tab.

Expected: the tab closes with no notification.
Actual: the tab closes and the error toast appears every time.

Cause (read from the code, not stepped through). the PR overview panel's refreshPanel() (minified in dist/extension.js, v0.174.0) checks this._panel.visible and this._refreshing once, then awaits resolveModel(...) and updateItem(...); its catch shows the toast:

async refreshPanel() {
  if (!(!this._panel.visible || this._refreshing)) {
    this._refreshing = true;
    try {
      const e = await this.resolveModel(this._identity);
      if (!e) throw new Error(`Failed to resolve Pull Request #...`);
      await this.updateItem(e);
    } catch (e) {
      window.showErrorMessage(`Error refreshing pull request description: ${...}`);
    } finally { this._refreshing = false; }
  }
}

After a merge the extension disposes the panel (PR done, branch deleted, default branch checked out) while a refresh that started earlier is still awaiting the model. When it resumes, updateItem posts to a disposed webview and throws.

Suggested fix. Track disposal (an onDidDispose flag, or re-check the panel after each await) and return quietly instead of calling showErrorMessage when the panel is gone. Related closed reports with the same message but other triggers: #2444, #4847, #5037.

Environment: extension 0.174.0, VS Code 1.144.0, Windows 11.