Output tab title becomes "undefined preview" and rendered HTML disappears after saving project

[floehopper]

Steps to reproduce:

1. Log in
2. View an HTML project, e.g. https://editor.raspberrypi.org/en/projects/anime-expressions-solution
3. Click the "Run" button
4. See that a tab appears in the output panel with the rendered HTML and the tab is titled "index.html preview"
5. Click the "Save" button
6. See that in the output panel the tab title has changed to "undefined preview" and the rendered HTML has disappeared

The tab title is obviously incorrect and I think it's a bit surprising the the rendered HTML disappears!

Screencast demo:

Screen.Recording.2024-06-05.at.12.37.10.mov