Errors when using 3rd-party blocks with Dynamic Sources since WP 5.9 update

Symptoms

Since WordPress 5.9, if using non-Toolset blocks (including core blocks) with dynamic sources in templates, Views, or custom archives, errors are thrown and it may not be possible to edit the page.

In the browser console errors such as the following may appear:

Uncaught TypeError: Cannot assign to read only property 'className' of object '#Object'
...

react-dom.js?ver=17.0.1:20115 The above error occurred in the  component:
...

Consider adding an error boundary to your tree to customize error handling behavior.

Workaround

We are addressing the issue and should have a fix in the upcoming releases.

In the meantime the errors only occur if your wp-config.php file includes:


define( 'SCRIPT_DEBUG', true );


Change that to false, or delete it, to resolve the problem in the meantime.