Gutenberg editor crashes when interacting with links in a Custom HTML block

Description

The Gutenberg editor encounters an unexpected error while updating a page that contains a Custom HTML block. The block includes a custom HTML structure with links to the current and external sites.

Step-by-step reproduction instructions

• Go to Pages -> Add New or edit an existing page.
• Add a Custom HTML block.
• Add a custom HTML structure containing one or more links, for example:

<div>
    Have questions? <a href="< add-link >" target="_self">Contact Us</a>
</div>

• Click the link in the Custom HTML block.
• The link attempts to load the linked page inside the editor iframe.
• Try to add a new block from the block inserter.
• Open the editor’s side panel.
• I am able to reproduce this issue in ​https://playground.wordpress.net/.

Screenshots, screen recording, code snippet

Here is the screen recording from WordPress Playground demonstrating the issue.

https://github.com/user-attachments/assets/70037180-1b92-47a0-bd27-6e0a890f350c

Environment info

• WordPress: 7.1
• Browser: Chrome v151.0.7922.169
• Theme: Twenty Twenty Five

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure

Reproduce the crash in WordPress Playground using a Custom HTML block with a link, then interact with the link, block inserter, and editor side panel. Start by tracing the Gutenberg editor behavior around the iframe after the link loads. Done means the editor remains usable and the block inserter and side panel open normally after interacting with the link.