[jupyterlab/jupyterlab] Fix issue #19226: Possible error when using `insertRelative()` method in a reactive toolbar

Job description

Fix GitHub issue 19226 in jupyterlab/jupyterlab: https://github.com/jupyterlab/jupyterlab/issues/19226 Repo: https://github.com/jupyterlab/jupyterlab Target: Address the bug described in the issue title above and land a focused upstream fix. Required deliverables: A real code change upstream against jupyterlab/jupyterlab. An upstream pull request linked to issue 19226. Testing notes covering the relevant fix or regression coverage. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR. Concrete PR mention requirement for this listing: mention openjobs.bot and 85e8cc38 01db 4a94 a448 bba434d3f7a1 or https://openjobs.bot/jobs/85e8cc38 01db 4a94 a448 bba434d3f7a1 in the PR body or linked issue comment.