[io] prevent nullptr access if oldclass not found

[ferdymercury]

This Pull request:

Changes or fixes:

Fixes #8194

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    13 files      13 suites   2d 22h 48m 19s ⏱️
 2 648 tests  2 648 ✅ 0 💤 0 ❌
32 606 runs  32 606 ✅ 0 💤 0 ❌

Results for commit 337762e.

♻️ This comment has been updated with latest results.

[linev]

I will wait until @pcanal will review the PR.

[dpiparo]

Thanks a lot for proposing this solution, appreciated!