Heading block "encountered an error" since 6.8.0

[niaccurshi]

Describe the bug
Upon installing the 6.8.0 plugin update, all of my heading blocks returned the "This block has encountered an error and cannot be previewed." error in the editor.

To reproduce
Steps to reproduce the behavior:

1. Have a site where the headings exist in the editor, with extra classes defined (unsure if it is relevant but all of my headings do have extra classes defined)
2. Upgrade the plugin to 6.8.0
3. Check the heading block and see the error returned

Expected behavior
Normal heading block display

[niaccurshi]

The same happens when adding a new heading block, the following error is created

[turansadri]

I had this in my theme setup (removes the colour palette) and noticed it now breaks the headings after 6.8.0 update.

add_theme_support( 'editor-color-palette' );

However it's easily fixed by giving an empty array instead of nothing.

add_theme_support( 'editor-color-palette', array() );

Hope this helps.

[niaccurshi]

Thanks this also resolved my issue, would be good to see an empty array applied as default to stop other themes crashing this way!

[alcarazzam]

Confirmed with a child theme.