Description

We are using a hybrid theme (theme.json but not a fully block theme). When testing out 6.9-RC2, clicking on Appearance-> Customize takes me to blank white screen. No errors are logged.

mysite.edu/subsite/wp-admin/customize.php?return=%subsite%2Fwp-admin%2F

Step-by-step reproduction instructions

1. Use a hybrid theme
2. Click Appearance -> Customize
3. See a blank white screen

Screenshots, screen recording, code snippet

No response

Environment info

No response

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